Toda-prime lower bound for certain square-free multiples of 3

From papers

Let T(n)T(n) be the set of Toda primes of nn, where a Toda prime is an odd prime pp satisfying

p14n,gcd(p,4np1)=1.p-1\mid 4n,\qquad \gcd\left(p,\frac{4n}{p-1}\right)=1.

Write t(n)=T(n)t(n)=|T(n)|. Toda-prime lower-bound conjecture. Let nn be an odd, square-free multiple of 33. Assume that there exists p{5,7,13}p\in\{5,7,13\} such that pnp\mid n, and that rnr\nmid n for r{5,7,13}{p}r\in\{5,7,13\}-\{p\}. Finally, assume there exists qT(3p){5,7,13}q\in T(3p)-\{5,7,13\} such that qnq\nmid n. Then

t(n)4.t(n)\geq 4.

This conjecture is the stated snag in the authors' attempted proof of the one-Toda-prime question. It supplies a conditional theorem-like lower bound for a structured family of integers, while the general existence question remains open.

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Sources & referencesView supporting material

Primary source

Stephen McKean, “Toda primes”, arXiv:2511.19744 (2025).

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