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An injective holomorphic function cannot have an essential singularity: Casorati-Weierstrass theorem would give values from a small punctured neighbourhood inside the open image of a disjoint neighbourhood, contradicting injectivity. Apply this at infinity to an injective entire function. A removable singularity there would make the function constant by Liouville's theorem. A pole makes it a polynomial by the Cauchy coefficient formula; its derivative has no zero, so the fundamental theorem of algebra forces degree one.

For an ordinary critical open string with all Neumann boundary conditions, there are 24 independent transverse creation oscillators at each positive mode number $n$. Summing their nonnegative occupation numbers gives one factor $(1-w^n{)}^{-24}$. The coefficient $d_p=[w^p]F(w)$ counts oscillator polarizations at level $p$, excluding continuous center-of-mass momentum and additional endpoint multiplicities. Cauchy coefficient formula gives $d_p=(2\pi i{)}^{-1}\oint F(w)w^{-p-1}dw$.

Write the Laurent series on the punctured disc as $f(z)={\sum }_{j=-\infty }^{\infty }{a}_j{z}^j$. A removable singularity means that $f$ extends holomorphically to the origin; exactly all negative-index coefficients vanish. A pole of order $k\ge 1$ means that $z^{k}f(z)$ extends with nonzero value at zero; exactly $a_{-k}\ne 0$ and every coefficient below $-k$ vanishes. An essential singularity is neither of these cases; exactly infinitely many negative-index coefficients are nonzero.

Here is a proof of the required Casorati-Weierstrass theorem consequence. If some disc of values around $w$ were missed in a punctured neighbourhood, then $|f(z)-w|\ge \epsilon >0$ there. The holomorphic function $q(z)=1/(f(z)-w)$ would be bounded, and hence have a removable singularity. If its extension had $q(0)\ne 0$, then $f=w+1/q$ would extend holomorphically. If $q(0)=0$, its zero has a finite positive order, since $q$ is not identically zero; thus $f$ would have a pole. Both contradict an essential singularity. Therefore every sufficiently small punctured neighbourhood has values arbitrarily close to every $w$. Choosing $0<|z_n|<\min (r/2,1/n)$ with $|f(z_n)-w|<1/n$ provesThe open mapping theorem states that a nonconstant holomorphic function on a domain maps open sets to open sets. If $f$ were injective with an essential singularity, choose a small open disc $U$ away from zero and a point $z_0\in U$. Its open image contains a disc about $w=f(z_0)$. The sequence just constructed has $z_n\to 0$ outside $U$, but eventually $f(z_n)\in f(U)$. Thus some $y_n\in U$ has $f(y_n)=f(z_n)$ although $y_n\ne z_n$, contradicting injectivity. An injective holomorphic function cannot have an isolated essential singularity.

For an injective entire function $g$, apply this to $f(z)=g(1/z)$. If its singularity at zero were removable, $g$ would be bounded outside a large disc, hence bounded everywhere and constant by Liouville's theorem. This contradicts injectivity. It must therefore be a pole, say of order $k$, so $|g(z)|\le C|z{|}^k$ for large $|z|$. For any Taylor coefficient of index $j>k$, the Cauchy coefficient formula gives $|a_j|\le C{R}^{k-j}$ on arbitrarily large circles; hence $a_j=0$. Thus $g$ is a nonconstant polynomial. Its derivative never vanishes by the allowed injectivity fact. If its degree exceeded one, the fundamental theorem of algebra would give a zero of this nonconstant derivative. Therefore injective entire functions are affine:

Count oscillator polarizations at fixed center-of-mass momentum. In a critical closed bosonic string, each chiral sector has 24 transverse oscillator species. A species at frequency $m$ can be occupied $0,1,2,\dots$ times, so its contribution is $(1-q^m{)}^{-1}$. The full chiral generating function is thereforeThe two sectors are independent, while closed-string level matching restricts their oscillator numbers to the same $N$. Thus the number of states at the closed-string mass level is $g_N=d_N^2$, not $d_{2N}$.

Put $q=e^{-\beta }$, with $\beta >0$. The Dedekind eta function obeysThe transformed argument $2\pi i/\beta$ has large imaginary part, so its product approaches one and $\eta (2\pi i/\beta )\sim e^{-{\pi }^2/(6\beta )}$. HenceThis establishes the singular exponential as well as its power prefactor. The corrections to the transformed eta product are exponentially small in $1/\beta$.

The Cauchy coefficient formula, followed by $q=e^{-\beta }$, gives a steepest-descent integral with leading formLet $n=N-1$. The saddle of $S(\beta )=n\beta +4{\pi }^2/\beta$ isAlong the vertical steepest-descent direction $\beta ={\beta }_{\ast }+it$, the quadratic part is $-S^{\prime \prime }({\beta }_{\ast })t^2/2$. Thus the saddle-point approximation gives the Gaussian prefactorThe point $q=1$ supplies the largest exponential contribution. At another fixed root of unity of order $k>1$, the corresponding modular singularity gives a smaller exponential $e^{4\pi \sqrt{n}/k}$, so it does not change this leading asymptotic. Squaring the chiral result gives the large-level degeneracy of a closed bosonic stringReplacing $N-1$ by $N$ gives the equivalent usual leading asymptotic. The power $-27/2$ matters: the two-sector degeneracy is not just an unspecified exponential.

Since $M=2\sqrt{(N-1)/{\alpha }^{\prime }}$, the exponential is $e^{{\beta }_{H}M}$ withHere Boltzmann's constant is one. Coarse-graining the levels into a density per unit rest mass introduces the Jacobian $dN/dM={\alpha }^{\prime }M/2$. ConsequentlyThis is distinct from the degeneracy per level, whose power in $M$ is $-27$. The internal-state contribution to a canonical partition function behaves at large mass asIt converges for $\beta >{\beta }_H$ and diverges for $\beta <{\beta }_H$. At $\beta ={\beta }_H$, this particular internal rest-mass integral converges because of the power $M^{-26}$; one should not discard the prefactor and claim divergence there without specifying the full thermodynamic ensemble. For example, including continuous momenta in 25 spatial dimensions multiplies the high-mass integrand by a factor proportional to $M^{25/2}$, still leaving convergence of this ideal one-string integral at the endpoint, although sufficiently high derivatives are singular. Multi-string effects, volume and interactions affect the detailed limiting thermodynamics.

The leading microcanonical entropy grows as $S(E)\sim {\beta }_{H}E$, with logarithmic corrections, so increasingly large energies are stored in increasingly excited, long strings rather than in an arbitrarily hot ordinary gas. The Hagedorn temperature is the boundary of canonical convergence from above in inverse temperature; extrapolating the free-string canonical description to $T>T_H$ fails. This counting does not settle the interacting phase above that scale. Also, the bosonic ground-state tachyon is a separate vacuum instability; the high-level asymptotic is meaningful as a formal spectrum calculation without asserting a stable bosonic thermal vacuum.

Start by using the metric equation of the Polyakov action to retain the Virasoro constraints after choosing conformal gauge. In the light-cone gauge in string theory, make $X^{+}$ proportional to worldsheet time on a patch with nonzero $k^{+}$. The constraints determine $X^{-}$ from the transverse coordinates, so its oscillators are not independent physical excitations. Neumann boundary conditions make each transverse coordinate a cosine standing wave on the open interval. Quantize these waves as independent harmonic oscillators and build their Fock space on the momentum-labelled vacuum. There are $d-2=24$ transverse directions in the critical bosonic string, and the normal-ordering intercept is one. Thus physical oscillator states have nonnegative occupation numbers $r_{n,i}$ and level $p={\sum }_{n,i}n{r}_{n,i}$, with $M^2=(p-1)/{\alpha }^{\prime }$. Continuous center-of-mass momentum is not part of the discrete oscillator degeneracy.

For each mode $n$ and transverse direction, summing its occupation numbers gives ${\sum }_{r\ge 0}{w}^{nr}=(1-w^n{)}^{-1}$. Therefore the open-string oscillator degeneracy generating function isThis is the Cauchy coefficient formula. The original PDF's power $w^{-p}$ is an index error: it extracts $d_{p-1}$, and would give zero for the vacuum $p=0$, which actually has one oscillator state. Any positively oriented circle strictly inside the unit disk works.

Put $w=e^{-t}$ with $t>0$. Expand the logarithm and sum the geometric series to obtainSince $t/(e^{mt}-1)\le 1/m$, dominated convergence gives $t\log F(e^{-t})\to 24{\sum }_{m\ge 1}{m}^{-2}=4{\pi }^2$. Thus the leading singular term is $4{\pi }^2/t$.

The subleading term printed in the PDF also needs correction: the product implies $+12\log ϵ$, rather than $-6\log ϵ$. To obtain the coefficient needed for the final power law, use $\tau =it/(2\pi )$ and the Dedekind eta function. Its definition gives $F(e^{-t})=e^{-t}\eta (\tau {)}^{-24}$. The modular identity $\eta (-1/\tau )=\sqrt{-i\tau }\eta (\tau )$, recorded in NIST's eta transformation formulas, and the product at large imaginary argument giveSince $t=-\log (1-ϵ)=ϵ+{ϵ}^2/2+O({ϵ}^3)$, this yieldsThe printed logarithmic correction cannot be used to derive the stated $p^{-27/4}$ prefactor; the corrected correction does.

For the saddle-point approximation, change the coefficient contour to $t=-\log w$. Its local contribution isSet $q=p-1$. The positive saddle is $t_0=2\pi /\sqrt{q}$, with $S(t_0)=4\pi \sqrt{q}$ and $S^{\prime \prime }(t_0)=q^{3/2}/\pi$. Along the local vertical steepest-descent direction $t=t_0+iy$, the exponent is $S(t_0)-S^{\prime \prime }(t_0)y^2/2$ to quadratic order. The factor $(t/2\pi {)}^{12}$ varies negligibly over its Gaussian width. HenceThis proves the large-level degeneracy of an open bosonic string, including the otherwise unspecified leading constant. The factor $p^{-6}$ comes from the eta prefactor and $p^{-3/4}$ from the saddle width. As allowed, no bound on the remote part of the contour is needed.

At high mass, $p\simeq {\alpha }^{\prime }{M}^2$, so the density grows exponentially as $e^{{\beta }_{H}M}$ within units $k_B=\hslash =c=1$. A canonical sum with $e^{-\beta M}$ cannot converge for $\beta <{\beta }_H$, regardless of polynomial prefactors. This suggests a Hagedorn threshold and a breakdown or transition of the ordinary dilute-string thermal description. The power prefactor can make the partition sum itself finite at the threshold while sufficiently high energy moments diverge; the asymptotic count alone does not prove an absolute upper temperature. These are properties of the formal high-level spectrum, separate from the tachyon instability of the bosonic vacuum.