OFFSET
1,1
COMMENTS
Composite numbers n such that 20^(n - 1) == 1 (mod n). - Michel Lagneau, Feb 18 2012
LINKS
EXAMPLE
20^20 = 104857600000000000000000000 == 1 (mod 21), so 21 is in the sequence.
20^22 = 41943040000000000000000000000 == 1 (mod 23), but 23 is actually prime, so it's not in the sequence.
20^24 = 16777216000000000000000000000000 == 0 (mod 25), so 25 is not in the sequence either.
MATHEMATICA
base = 20; pp20 = {}; n = 1; While[Length[pp20] < 100, n++; If[!PrimeQ[n] && PowerMod[base, n - 1, n] == 1, AppendTo[pp20, n]]]; pp20 (* T. D. Noe, Feb 21 2012 *)
searchMax = 10000; Complement[Select[Range[searchMax], PowerMod[20, # - 1, #] == 1 &], Prime[Range[PrimePi[searchMax]]]] (* Alonso del Arte, Feb 20 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved