OFFSET
1,21
LINKS
Michel Marcus, Table of n, a(n) for n = 1..1000
EXAMPLE
For n=5 we have (2^0) mod 5 = 1, (2^1) mod 5 = 2, (2^2) mod 5 = 4, (2^3) mod 5 = 3, (2^4) mod 5 = 1. Only for s=3 does (2^s) mod 5=s, so a(5)=1
MATHEMATICA
Table[Count[Range[0, n - 1], _?(Mod[2^#, n] == # &)], {n, 105}] (* Michael De Vlieger, Jun 19 2018 *)
Table[Count[Range[0, n-1], _?(PowerMod[2, #, n]==#&)], {n, 110}] (* Harvey P. Dale, Aug 02 2024 *)
PROG
(PARI) a(n) = sum(s=0, n-1, Mod(2, n)^s == s); \\ Michel Marcus, Jun 19 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Jonathan Ayres (jonathan.ayres(AT)btinternet.com), Oct 28 2001
EXTENSIONS
a(1) corrected by Michel Marcus, Jun 20 2018
STATUS
approved