OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
F. Luca and V. Tipu, On positive integers n with a certain divisibility property, J. Integer Sequences 12 (2009), 11 pp.
FORMULA
n log n << a(n) << n^(1+e) for any e > 0. See Luca & Tipu for more precise results. - Charles R Greathouse IV, Feb 01 2013
EXAMPLE
For n=2, the second number is a(2)=10 because s(10)=2+5=7 divides 2^{10-1}-1=2^9-1=511.
MAPLE
B := {}; for n from 2 to 1000 do A := (numtheory[factorset])(n); b := add(a, `in`(a, A)); if `and`(b < n, `mod`(2^(n-1), b) = 1) then B := [op(B), n] else end if end do; print(c := 2);
MATHEMATICA
Select[Range[2, 2300], CompositeQ[#] && PowerMod[2, #-1, Total[First /@ FactorInteger[#]]] == 1 &] (* Amiram Eldar, Nov 20 2019 *)
PROG
(PARI) is(n)=if(isprime(n), 0, my(f=factor(n)[, 1]); Mod(2, sum(i=1, #f, f[i]))^(n-1)==1) \\ Charles R Greathouse IV, Feb 01 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Florian Luca (fluca(AT)matmor.unam.mx), Feb 15 2009
EXTENSIONS
More terms from Amiram Eldar, Nov 20 2019
STATUS
approved