OFFSET
1,2
COMMENTS
Also, numbers m such that (4^k)^(m-1) == 1 (mod (m-1)^2 + 1) for all k >= 0.
a(20) > 2*10^12, if it exists. - Giovanni Resta, Feb 26 2020
FORMULA
a(n) = sqrt(A273999(n)-1) + 1. - Jinyuan Wang, Feb 24 2020
EXAMPLE
5 is a term because 4^(5-1) == 1 (mod (5-1)^2+1), i.e., 255 == 0 (mod 17).
PROG
(Magma) [n: n in [1..100000] | (4^(n-1)-1) mod ((n-1)^2+1) eq 0]
(PARI) isok(n) = Mod(4, (n-1)^2+1)^(n-1) == 1; \\ Michel Marcus, Jun 02 2016
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jaroslav Krizek, Jun 01 2016
EXTENSIONS
a(14)-a(15) from Michel Marcus, Jun 02 2016
Edited by Max Alekseyev, Apr 30 2018
a(16)-a(19) from Jinyuan Wang, Feb 24 2020
STATUS
approved