A241977 Numbers k>1 such that 10^phi(k) == 1 (mod k^2), where phi(n)=A000010(n).

3, 487, 1461, 4383, 13149, 39447, 118341, 355023, 56598313, 169794939, 509384817, 1754547703, 5263643109, 7187985751, 15790929327, 21563957253, 27563378431, 33902389487, 47372787981, 50315900257, 64691871759, 82690135293, 101707168461, 150947700771

Offset: 1

Author

Felix Fröhlich, Aug 10 2014

Keywords

• nonn

Links

• Giovanni Resta, Table of n, a(n) for n = 1..130 (terms < 10^15)

Crossrefs

Cf. A000010, A045616, A077816, A171928, A242958, A242959, A242960, A245529.

Programs

Mathematica

Select[Range[400000], PowerMod[10, EulerPhi[#], #^2] == 1 &] (* Amiram Eldar, Oct 16 2023 *)

PARI

for(n=2, 1e9, if(Mod(10, n^2)^(eulerphi(n))==1, print1(n, ", ")))

Extensions

Terms a(12) and beyond from Giovanni Resta, Jan 24 2020