The 26th power of the Dedekind $\eta$ function has been mentioned several times here on MO:
A 14th and 26th-power Dedekind eta function identity?
and in comments to some other questions and answers.
Specifically, there seems to be some (unpublished?) work by Atkin on this. In "Missed opportunities" (1972) Dyson says in his reference 13
A.O.L. Atkin had known of these formulæ for some time before 1968 when he sent me proofs of them. His work on them is still unpublished.
and reproduces, in reference 14, a(n astounding) formula which he says
...is a special case of Atkin's formula for $d=26$.
In "The 26th power of Dedekind's $\eta$-function" by Chan, Cooper and Toh (2006) one finds
Atkin’s notes [1] indicate that he discovered his formula for $\eta^{26}(z)$ in 1965, and in 1966 he found another formula, different from the one quoted by Dyson
Their reference [1] reads
A.O.L. Atkin, E-mail to S. Cooper, September 29, 2004.
Other than that, I found in an answer to the second of those linked MO questions this: in an appendix to a Russian translation of "Missed Opportunities" (paywall, sorry) by Monastyrsky (1980), it is said that formulæ for $\eta^{26}$ and $\eta^{52}$ can be obtained using Macdonald identities for $F_4$. No details or citations are given at that place; a paper by Atkin, namely "Ramanujan Congruences for $p_{-k}(n)$" (1968) is in the references, but I could not find there anything about formulæ for $\eta^{26}$. In "Sur la lacunarité des puissances de $\eta$" (1985) Serre has an expression for $\eta^{26}$ and also suggests looking at $F_4$ but does not mention Atkin. Also in Toh's thesis (2007) containing another proof of the identity of Chan, Cooper and Toh mentioned above the reference is just "A. O. L. Atkin. Unpublished".
That's all I could find. Does anybody know whether Atkin's work can be accessed somehow?
