OFFSET
1,8
EXAMPLE
For n = 8: A002808(8) = 15 and b^(15-1) == 1 (modulo 15) for three values of b with 1 < b < c, namely 4, 11, 14, so a(8) = 3.
MATHEMATICA
DeleteCases[Table[If[CompositeQ@ n, Count[Range[2, n - 1], b_ /; Mod[b^(n - 1), n] == 1], -1], {n, 117}], -1] (* Michael De Vlieger, May 09 2017 *)
PROG
(PARI) forcomposite(c=1, 200, my(i=0); for(b=2, c-1, if(Mod(b, c)^(c-1)==1, i++)); print1(i, ", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
Felix Fröhlich, May 09 2017
STATUS
approved