OFFSET
1,2
COMMENTS
Serret proved in 1855 a generalization of Fermat's little theorem: for b >= 1, Sum_{d|k} mu(d)*b^(k/d) == 0 (mod k). This sequence includes numbers k such that k^2 divides the sum with base b=2.
Includes all the powers of 2 above 8.
An alternative generalization of Wieferich primes (A001220) which are the prime terms of this sequence.
Also numbers k such that k | A059966(k).
REFERENCES
Wacław Sierpiński, Elementary Theory of Numbers, Elsevier, North Holland, 1988, page 217.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..250
Joseph-Alfred Serret, Théorème de Fermat généralisé, Nouvelles Annales de Mathématiques, Vol. 14 (1855), pp. 261-262.
EXAMPLE
16 is in the sequence since Sum_{d|16} mu(d)*2^(16/d) = 65280 = 255 * 16^2.
MATHEMATICA
f[n_] := DivisorSum[n, MoebiusMu[#] * 2^(n/#) &]; Select[Range[1000], Divisible[f[#], #^2] &]
PROG
(PARI) isok(n) = frac(sumdiv(n, d, moebius(n/d)*(2^d-1))/n^2) == 0; \\ Michel Marcus, Jul 30 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Amiram Eldar, Jul 29 2018
STATUS
approved