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Past exam of the mathematics course of the University of Cambridge 2010 iii Paper 26 1 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
Let with an algebraic integer, let be its monic minimal polynomial of an algebraic element, and write . The Kummer-Dedekind theorem applies to a rational prime not dividing the finite index . Ifwith distinct monic irreducible polynomials , choose monic integer lifts . Then the distinct prime ideals above areIn particular, is unramified precisely when has no repeated irreducible factor. The exclusion of primes dividing the index is essential.
Here is the quotient-ring proof of the Kummer-Dedekind theorem. The finite group has order coprime to . Localizing at the integers coprime to therefore gives , and reduction yieldsThe second isomorphism follows from . The Chinese remainder theorem givesEach factor is a local ring with unique maximal ideal generated by and residue field . Pulling its maximal ideal back to gives exactly ; these exhaust the primes above . This proves the asserted residue degrees and their independence of the chosen lifts.
It remains to prove the exponents rather than simply identify the primes. Unique prime ideal factorization in the Dedekind domain writes . At , its discrete valuation ring has , with a unit. Thus the maximal ideal of has nilpotence index of an ideal : its th power is zero and its st is nonzero. In this index is , since but . The displayed product isomorphism identifies these local factors, hence . This completes the theorem's proof. The same local argument works over a base number field whenever equals the full relative ring of integers after localizing at the chosen base prime.
For the required cyclotomic field, the allowed ring of integers formula makes the index one. Its cyclotomic polynomial isFor , the polynomial has derivative coprime to it modulo . Its divisor is therefore squarefree, so the Kummer-Dedekind theorem gives no ramification.
The cyclotomic reduction at a conductor prime is explicit at the remaining two primes:The quartic over has no linear factor. Its only possible irreducible quadratic divisor would be , whose roots have order three, whereas its own roots have order five. Hence it is irreducible. The two quartics over are also irreducible: their roots have multiplicative order sixteen because their fourth powers are or , both of order four; the finite-field Frobenius automorphism has orbit length four on an element of order sixteen, since and neither nor is modulo sixteen.
Consequently the prime ideal factorization above has one prime with ramification index eight and residue degree four. Above it has two primes, each with ramification index four and residue degree four. In particular,
Past exam of the mathematics course of the University of Cambridge 2010 iii Paper 25 5 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
The rational approximation below is taken in lowest terms, , with , and . This is necessary for denominator-sensitive exponential sum estimates and for the final Ramanujan sum to equal . If , the defining sum over is empty and ; hence we may assume .
Set , so , and write the given Vaughan identity term as the Type II sumUse a dyadic decomposition into , , retaining . Empty blocks may be discarded; every remaining block has , , and there are blocks. Denote a block by . The Cauchy-Schwarz inequality givesBy the allowed divisor-square summatory bound, the first factor is . Expand the second factor. The diagonal contributes , using . For a pair , the allowed form the intervalThus the exponential geometric sum bound applies to this interval even though the original block has a hyperbolic cutoff. For each difference , there are ordered pairs, giving an upper boundfor the expanded second factor, with an absolute constant accommodating integer endpoints. The supplied reciprocal fractional-part sum near a rational, with and , bounds the last sum byOne may also derive this directly from the separated reciprocal-distance sum by splitting into blocks of length at most ; the approximation error between two block points is at most , and the rational rotations are separated by .
Combining the estimates yieldsSince and , both . Taking square roots provesSumming the dyadic blocks establishes the rational-phase Type II estimate and, in particular, the requested bound with an explicit logarithmic power:
The Siegel–Walfisz theorem states that for fixed , uniformly for and ,The constant is allowed to be ineffective. Apply this with to the reduced residue classes. Their contribution to the prime exponential sum issince there are at most residue classes. To evaluate the Ramanujan sum, the Möbius divisor-sum identity gives . Then the finite geometric series givesWhen , only remains, giving . The omitted nonunits contribute only prime powers with . For each such prime, the sum of their Von Mangoldt function weights is at most , so their total is at most , absorbed by the stated error. This proves the major-arc value of the prime exponential sum:The original PDF does not state coprimality in this last request. Without it the asserted main term is genuinely false: makes and hence , whereas . For an arbitrary numerator the correct main term is , with given by the divisor formula above. Reducing the fraction before applying the boxed result is the equivalent repair.
Past exam of the mathematics course of the University of Cambridge 2010 iii Paper 25 4 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
Write and use for distance to the nearest integer. The Dirichlet approximation theorem, in its pigeonhole form, says that for every real and integer there are coprime integers with and . Indeed put the fractional parts of into equal intervals; two lie in the same interval. Their difference gives , and reducing the fraction preserves the claimed bound. This includes rational .
For , the finite geometric series has length . When ,The last inequality uses the concavity of sine on . The triangle inequality also gives . At integer the sum has absolute value , interpreting the reciprocal as infinity. Thus the precise exponential geometric sum bound isThe interval must be nonempty; for its sum is zero and its length should be defined as , rather than using a potentially negative bound.
For the separated points, assume and . An arc of length contains at most of the points, by ordering them along the arc. Points with contribute . In the dyadic annulus , each summand is and there are at most points. There are nonempty annuli, giving the stronger separated reciprocal-distance sumIf there is only one point the bound is immediate; otherwise . The printed requires a lower bound away from ; avoids that endpoint defect.
A useful version of the quadratic Weyl inequality is the following, with , and :For , expand the square of this quadratic exponential sum by :The exponential geometric sum bound therefore gives . For , partition these into blocks of length at most . Two different points in one block have difference with . The reduced denominator of is at least , so , whileThus the rotations are separated by at least in each block. The separated reciprocal-distance sum bounds each block by . There are blocks, and henceFor or , the trivial bound supplies the same assertion with an absolute constant. The same proof applies to , whose difference sums also have interval endpoints.
We finally prove a polynomial rate of quantitative quadratic recurrence, without using the optional finite-group assumption. For , use the periodic triangular bump . It is times the periodic convolution of two interval indicators, and therefore has the nonnegative Fourier coefficientsIts Fourier series is absolutely convergent, , and the coefficient tail is . Suppose that for every . Then . Choose with a sufficiently large absolute , so that the contribution of frequencies has absolute value at most . The zero frequency contributes . Since all coefficients are nonnegative and their sum is one, some must satisfyApply the Dirichlet approximation theorem to with , obtaining a reduced , . The quadratic Weyl inequality then impliesSet . For sufficiently large , the term is absorbed on the left, giving . The integer consequently satisfiesMoreover the approximating integer is , andThis contradicts the assumed avoidance. Increasing the absolute constant handles the finitely many smaller , uniformly for every real . Thus
Past exam of the mathematics course of the University of Cambridge 2010 iii Paper 25 3 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
For , the Euler products areThe Dirichlet character is completely multiplicative, and unique prime factorization expands each finite product into the terms of the corresponding Dirichlet series supported on those primes. Absolute convergence permits passage to all primes. More strongly, the absolutely convergent Euler product logarithm givessince the sum of absolute values is bounded by .
We prove nonvanishing of a nonprincipal Dirichlet L-function at one, including real Dirichlet characters. If is nonprincipal modulo , periodicity and the given Orthogonality of Dirichlet characters bound by . Partial summation giveswhich is holomorphic for by locally uniform convergence, including after differentiation. The principal Dirichlet character instead haswith a simple pole at one.
For a nonreal Dirichlet character, consider . For real , its Euler product logarithm isThe given character inversion with makes each inner sum either or zero, so . If for a nonreal Dirichlet character, then as well, since their values at real are conjugate. These are two distinct factors. All nonprincipal factors are holomorphic at one, and only the principal factor has a simple pole. Their two zeros would force , a contradiction.
For a real nonprincipal Dirichlet character, suppose . Then is holomorphic throughout : the supplied continuation of the Riemann zeta function has no other pole there, and the zero cancels the pole at one. On its Dirichlet series has coefficientsTo check the sign explicitly, for a prime power the local sum is if , is for even and for odd if , and is if . Multiplication over prime factors proves nonnegativity and . Also , so termwise derivatives at are justified by absolute convergence:The disc lies in . Its Taylor series therefore converges at , and Tonelli theorem givesThis contradicts holomorphy at . It is the positive-coefficient argument behind the Landau theorem for a Dirichlet series with nonnegative coefficients, proved here rather than invoked. Thus for every nonprincipal Dirichlet character.
It remains to detect the primes. For real , the Euler product logarithm satisfiesuniformly as : terms with are bounded in absolute value by . For a nonprincipal Dirichlet character, a holomorphic logarithm of exists near one by the nonvanishing just proved. On a short real interval it differs from the Euler product logarithm by a fixed multiple of , so the prime sum stays bounded. For the principal Dirichlet character, the simple pole instead givesApplying the supplied Orthogonality of Dirichlet characters for a reduced residue class now yieldsThe right side tends to infinity, whereas a finite set of primes would give a bounded left side. Therefore every residue class coprime to contains infinitely many primes, proving the Dirichlet theorem on primes in arithmetic progressions.
Past exam of the mathematics course of the University of Cambridge 2010 iii Paper 25 2 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
Use the Fourier transform convention . The complex measure is finite because . The Möbius divisor-sum identity, proved in the preceding solution, allows multiplication of absolutely convergent Dirichlet series and givesHere is the Riemann zeta function. We prove the estimates in the intended range . A uniform estimate of the printed form for all would be false: at , as , while its proposed upper bound tends to zero. A version valid for every replaces by .
For completeness, unique prime factorization and absolute convergence give andas the logarithm defined by that Euler product. The elementary nonnegative trigonometric polynomialtherefore proves the three-four-one inequality for Euler productsWe also need elementary uniform bounds for derivatives of the Riemann zeta function. For an integer , summation by parts givesThis continues the function to with just the pole at one. For , , choose . Differentiating times, the finite Dirichlet polynomial is bounded by . Derivatives of the second term have the same bound, since . In the last term, differentiated integrands are bounded by , whose integral is ; multiplication by is harmless. It follows thatAs , the three-four-one inequality for Euler products gives, for ,To handle bounded , first note that cannot vanish at for : a zero of order would make the three-four-one product , contradicting its lower bound one. At , the reciprocal extends holomorphically with a simple zero, since the pole of has residue one. Thus and all its derivatives are bounded on the compact rectangle , . This proves the requested reciprocal zeta bounds near the line one:
For the derivative estimate, repeatedly differentiate . Every term of is a constant timesThe preceding derivative bounds and reciprocal estimate bound this by . Compact-height bounds supply the remaining range. Since , this has exactly the requested dependence, with an explicit logarithmic exponent linear in for .
Here is the unsmoothing a logarithmically weighted sum argument with the normalization repaired. For real ,Indeed the Fourier transform of is , so Fourier inversion gives the identity, also at by continuity. In a vertical contour integral the coefficient is and the differential is . The printed real- integral has with and is missing this factor of .
Set , taking when . Its Dirichlet series is . The logarithmically smoothed Perron formula, justified by absolute convergence, givesFor choose . The integral of is finite, with a constant depending only on , and . Thus, with ,For , subtraction gives the exact finite-difference identityThe last sum is , since there are terms. ConsequentlyFor , choose to get exponent . For , the trivial bound is already no larger than that estimate. The extra term is absorbed for fixed . HenceThis also yields the cancellation needed for the Prime number theorem. Choose a fixed , discard at cost for the unweighted sum, and use partial summation with on the rest. The displayed bound gives ; the preceding solution then gives .
Past exam of the mathematics course of the University of Cambridge 2010 iii Paper 25 1 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
Write and let be the Identity for Dirichlet convolution. The Möbius function uses the empty-product convention ; the Von Mangoldt function has . We first establish all the arithmetic function identities needed below. Every integer divisor of contributes one to , so . If has distinct prime factors, the squarefree divisors correspond to their subsets andAt the sum is one. Thus . Writing , the only divisors with nonzero Von Mangoldt function are , and henceThe Dirichlet convolution is commutative by replacing with , and associative because either bracketing sums over . ConsequentlyThese are consequences of the definitions, rather than unproved invocations of Möbius inversion.
To identify the constant, integrate on each interval :The tail is , since the integrand is at most for . Summation therefore gives , where is the harmonic number, and . Thus this is Euler's constant.
Put . Counting pairs of positive integers whose product is at most , and subtracting the square counted twice, gives the Dirichlet hyperbola method identitySince , the harmonic number estimate yieldsThe increasing natural logarithm also satisfies by comparison with its integral. Subtracting proves
For the second implication, denote the Mertens function by and the Second Chebyshev function by . Apply the proved Dirichlet convolution identities to :Summing this identity over integers up to gives the exact relationFix an integer . The contribution to from has absolute value at mostFor , group by . This gives exactlyunder the assumed cancellation ; extending that assumption from integers to real uses . Therefore . Letting proves the prime number asymptotic from Mertens cancellation:
Past exam of the mathematics course of the University of Cambridge 2010 iii Paper 23 5 Solution by
Codex 0 Created 2026-10-03 Updated 2026-10-14
A one-variable main conjecture for CM elliptic curves compares a torsion arithmetic Iwasawa module with a measure interpolating special Hecke L-function values. The classical bounded-measure formulation uses a prime of good reduction which splits in the CM field, hence gives ordinary reduction. We first describe the one-prime division-tower formulation, including the analytic construction, and then explain its cyclotomic Selmer counterpart and the change at supersingular primes.
For a precise basic setting, take with complex multiplication by , of class number one, and a split prime of good reduction, prime to the Hecke conductor . Write , choose the -adic embedding corresponding to , and let . Its formal group of an elliptic curve has height one, with . The one-prime division towerhas of order prime to and . Its CM character describes the action on the prime-primary Tate module. Let and . We extend coefficients to the complete unramified ring to include character values and a CM period.
Let be the Galois group over of the maximal abelian pro- extension unramified outside primes above . Let be the analogous everywhere-unramified group, equivalently the norm inverse limit of the -parts of the ideal class groups. Write for the norm limit of principal local units at , for the closures of global units in , and for the closures of elliptic units. Roots of unity and finite initial-layer errors are retained when formulating integral statements; after passage to a finite-character component they do not change a characteristic ideal.
The class-field unit sequence, obtained by taking norm limits in global Artin reciprocity, givesIn this ordinary one-prime tower, the unit and global-unit terms have the same Iwasawa rank, and the displayed quotients and are torsion on their character components. The equalityis the global-units/class-groups form of the CM main theorem. Multiplicativity of characteristic ideals in the sequence then givesThe analytic construction must identify the latter ideal with a genuinely interpolating p-adic L-function; it is not enough to rename an arbitrary characteristic power series as an L-function.
Here is that construction. For an auxiliary ideal prime to , a suitably normalized rational elliptic function has divisor a multiple ofTranslate by a generator of the conductor torsion and take the product over its ray-class orbit. This produces a rational function defined over which is a unit series at the identity. Evaluating it on compatible -division points gives elliptic units. The reason for the norm relations is geometric: multiplication by groups the translated zeros and poles into fibers, so the product over has the divisor of . The normalization fixes the constant, giving the local identityThis is also the norm relation of the Coleman power series for a CM formal group interpolating the resulting local units.
An integral logarithmic correction makes the passage from those units to a bounded measure explicit. In the chosen split completion , putSince and the coefficients of reduce to , the ratio is in . For , its logarithm divided by is integral. Constants are treated using the usual -adic logarithm on units. This proves the integrality needed for the Frobenius-corrected logarithm; applying an uncorrected logarithm alone would not prove boundedness.
Choose an isomorphism over , and normalize the CM period byThen is the Amice transform of a unique -valued p-adic measure on . The norm relation above givesthe first logarithmic sum is and the second is the same sum, because and there are points. On the measure side the sum selects and multiplies by , so this identity says that the measure is supported on . Transport it to by the CM character and project to the desired component.
The moments can now be calculated rather than postulated. With ,The factor appears because is multiplication by in the additive formal-logarithm coordinate. The remaining derivative is a Coates–Wiles homomorphism. The complex product for the elliptic function makes it a sum of shifted inverse powers of lattice elements. Grouping these lattice elements by ray-class ideals identifies that sum with , multiplied by the auxiliary factor . Here omits primes dividing the original conductor, including when the conductor of the power character becomes smaller.
Normalize the rational functions and matching complex and -adic periods consistently, and remove the auxiliary factors by the distribution relations. They are evaluations of in the Iwasawa algebra. On each finite-character component an auxiliary ideal can be chosen making this element a unit: its residues at and can be prescribed independently by the Chinese remainder theorem. The resulting integral measure satisfies, on its branch and with in the corresponding residue class,The exact normalization of periods matters for this equality; multiplication by an integral unit does not affect the ideal in the main conjecture. Finite-order twists are obtained by inserting a character in the ray-class sums, with its local Gauss sum. The lattice calculation proves algebraicity of the normalized complex values, while the integral series construction proves their -adic interpolation. These are different steps.
The Coleman measure map identifies with a rank-one Iwasawa algebra lattice, up to finite errors, and takes to the ideal generated by the corresponding power series . The conjecture therefore has the concrete formIts proof uses the Euler system of elliptic units. Adjoining auxiliary ray moduli supplies norm relations with factors . Derivative operators in the auxiliary cyclic groups produce Kolyvagin classes; their controlled valuations relate a new auxiliary prime to its ideal class. Chebotarev density theorem permits the required Frobenius choices. Successively choosing these primes bounds the class-group elementary divisors by the elliptic-unit index, giving the difficult characteristic-ideal divisibility. The opposite comparison comes from the class-number/unit-index formula and passage through the tower. Their equality, combined with the exact sequence above and the logarithmic computation, is the proof mechanism. The ordinary split-prime setting of this formulation is specified in Rubin's one-variable main-conjecture paper.
For completeness, the familiar cyclotomic formulation concerns a different tower. If has complex multiplication and good ordinary reduction at , put andFor a finite layer , its Selmer group of an elliptic curve iswhere contains the bad primes and primes above ; at infinite level take the direct limit. Thus it imposes the local Kummer conditions and sits between rational points tensored with and the -primary Tate–Shafarevich group. Its dual is a torsion Iwasawa module, and the cyclotomic main conjecture isWith compatible reciprocity conventions, the ordinary function has the finite-character interpolationwhere is the -adic unit root of and ; characters of this cyclotomic -extension are even. The two-variable CM measure specializes to this line after the CM-character twist. Kummer theory, class-field descriptions of the prime-primary local conditions and control of finite-layer errors identify its algebraic counterpart with the displayed Selmer dual. The one-prime division tower is not itself the cyclotomic tower, so this requires the twist and specialization argument, not just changing its name.
Finally, at a good inert prime the curve has supersingular reduction. The ordinary unit root is absent, the unmodified cyclotomic Selmer dual is not torsion, and the ordinary bounded-function argument does not apply. For CM curves over and odd supersingular , signed local trace conditions and signed functions give instead , as proved in Pollack and Rubin's supersingular main-conjecture theorem. Thus the ordinary hypotheses and the analytic local correction are essential parts of the essay's main-conjecture statement.
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