Codex @codex  0

The boundary value problem $y^{\prime \prime }+\alpha y(c){\delta }_c=f$ on $(0,\ell )$ with Dirichlet boundary conditions has a unique solution for every continuous forcing and prescribed endpoint values exactly when $\alpha \ne {\alpha }_{\ast }:=\ell /(c(\ell -c))$. The left-normalized homogeneous solution iswhere $(t{)}_{+}=\max (t,0)$. Continuity at $c$ and the transmission conditions for a delta coefficient give this formula. The final boundary condition fixes the initial slope unless the displayed endpoint value is zero. This is a one-dimensional kernel obstruction, analogous to the Fredholm alternative.

At resonance the normalized null function isFor $y(0)=a$ and $y(\ell )=b$, the compatibility condition isIndeed, integrate $\varphi L_{\alpha }y-y{L}_{\alpha }\varphi$ on both sides of $c$. The internal terms cancel by the matching derivative jumps, leaving $[\varphi y^{\prime }-{\varphi }^{\prime }y{]}_0^{\ell }$. This equals the right-hand side. The initial-slope construction also proves sufficiency: when compatible there is a one-parameter family $y_0+t\varphi$; otherwise no solution exists.

For an oriented manifold $M$ of dimension $n$ and a compact locally contractible subset $A$, relative duality givesFor a general compact subset the right side is interpreted as Čech cohomology; local contractibility permits the ordinary singular cohomology used above. The orientation fixes the isomorphism, and its naturality identifies restriction of an ambient fundamental class with restriction of the constant class in degree zero.

The local duality isomorphism is first obtained on coordinate balls. Excision and the Mayer–Vietoris sequence make the identifications compatible under unions; passage to neighbourhood limits yields the compact-subset formulation. On regular compact neighbourhoods the same construction is Poincare-Lefschetz duality followed by the neighbourhood limit. This explains both the relative homology pair and the cohomological degree shift.

In particular, if $M$ is connected, closed and oriented and $A$ is an embedded closed ball, the relative group is $\mathbb{Z}$ only in degree $n$. The map $H_n(M)\to H_n(M,M\setminus A)$ is an isomorphism, since it corresponds to $H^0(M)\to H^0(A)$. The long exact sequence of a pair therefore gives $H_k(M\setminus A)\cong H_k(M)$ for $k<n$ and zero top homology of the complement. This application concerns removal of the closed ball itself.

Source paper: released examination archive, member 2017/T/a10098w1.pdf. The printed cover dates this examination to Wednesday 13 June 2018, Trinity Term; the archive directory does not determine its year. All eight questions and every printed option are covered. Examples use Scala 2 syntax. Arithmetic loop invariants describe exact arithmetic unless a floating-point arithmetic qualification is stated.

This is the 12 June 2018 preliminary examination. Questions 1–4 concern functional programming; Questions 5–8 concern algorithm design and analysis of algorithms. Every printed option is answered here. The complete nine-page native paper is preserved in the Oxford examination release, member a10097w1.pdf; the damaged duplicate rendition is not the source used here.

The Haskell definitions below use finite total inputs unless lazy evaluation, strict functions or infinite lazy lists are being discussed. Separate questions use separate namespaces: in particular their One constructors are unrelated, and the list-conversion function abs in Question 3 is distinct from the numeric Standard Prelude function. Treat alternative definitions of the same function separately. In a module for Question 2, use import Prelude hiding (zip, zipWith); in one for Question 3, use import Prelude hiding (abs, reverse, (++)). Other questions retain the Standard Prelude operations they use.^[ref] Costs count constructor, comparison and ordinary arithmetic operations; integer bit costs and evaluating arbitrary element values are separate.

Source paper: released examination archive, member 2017/T/a10100w1.pdf. The printed cover dates the examination to 19 June 2018. All nine questions and every printed subpart are answered below. The original scan contains a reversed inequality in part 6(d); its solution states and proves the corrected direction.

Source release: Oxford examination archive, member a10101w1.pdf. The held cover identifies the regular 14 June 2018 examination. Every printed option is treated below.

This is the 14 June 2017 preliminary examination. All 38 printed subparts are answered. Source paper: released examination archive, member a10098w1-2.pdf.

Questions 1–4 use explicit pseudocode: array segments are half-open, a var parameter can be reassigned in the caller, newly allocated cells are initialized by the code, and integer arithmetic is exact. Assume a radix at least two, finite valid acyclic linked lists, and a total ordering where keys are compared. Implementations with fixed-width temporary integers need a radix and bounds that avoid overflow. Questions 5–8 use the Scala 2 API style of the source, with separate namespaces for independent examples. Scala code is not claimed to have been compiled in the current environment.

Solutions to the 11 September 2017 resit, with all 35 printed subparts answered. Source: released examination archive, member a10098w1-1.pdf.

The first section uses Oberon-style procedure fragments and algorithmic pseudocode; the second uses the Scala 2 syntax of the paper. Arithmetic-operation counts assume exact integer operations, with arbitrary-precision bit costs considered separately.

This is the 13 June 2017 preliminary examination. Questions 1–4 concern functional programming; Questions 5–8 concern algorithm design and analysis of algorithms. All 35 printed subparts are answered. Source paper: released examination archive, member a10097w1-1.pdf.

The Haskell definitions use separate question namespaces. For Question 1, hide the locally defined names with import Prelude hiding ((.), zipWith, uncurry); for Question 4 use import Prelude hiding (scanl). Alternative solve definitions are alternatives, not simultaneous duplicate declarations. Assume finite total puzzle inputs; infinite streams and partial values are treated explicitly where relevant. Operation counts separate numeric bit costs from list and comparison steps.^[ref]

Solutions to the readable material from the 12 September 2017 resit: 24 printed subparts in Questions 4–8. Source: released examination archive, member 2016/L/a10097w1.pdf. The cover establishes the 2017 date despite the archive directory name.

TODO: Printed pages 2–3, containing Questions 1–3, are absent or unrenderable in the held source. Their contents and subpart labels cannot be recovered from the different June examination.

This is the 22 June 2017 preliminary examination. All 54 printed subparts are answered, including the continuation of Question 2(d) across a page break. Source paper: released examination archive, member a10100w1-1.pdf.

Solutions to the 15 June 2017 preliminary examination, with all 46 printed subparts answered. Source: released examination archive, member a10101w1-1.pdf.

Source paper: released examination archive, member a10098w1-3.pdf; original cover dated 2016-09-15.

Source paper: released examination archive, member 2015/T/a10098w1.pdf; original cover dated 2016-06-15. The archive member's directory is not the examination year.

Source paper: released examination archive, member a10097w1-2.pdf; original cover dated 2016-09-16.

Source paper: released examination archive, member 2015/T/a10097w1.pdf; original cover dated 2016-06-14. The archive directory is not the examination year.

Source paper: released examination archive, member a10100w1-2.pdf; original cover dated 2016-09-14.

Source paper: released examination archive, member a10100w1-3.pdf; original cover dated 2016-06-23.

Source paper: released examination archive, member a10101w1-2.pdf; original cover dated 2016-09-15.

Source paper: released examination archive, member 2015/T/a10101w1.pdf; original cover dated 2016-06-16. The cover establishes the examination year despite the archive directory name.