OFFSET
1,2
LINKS
Paolo P. Lava, Table of n, a(n) for n = 1..10000
EXAMPLE
sigma(10) = 18, 10*11/2 - sigma(10) = 55 - 18 = 37 and gcd(18,37) = 1 because 18 = 2*9 and 37 is prime.
MAPLE
with(numtheory): P:=proc(q) local n; for n from 1 to q do
if gcd(sigma(n), n*(n+1)/2-sigma(n))=1 then print(n); fi; od; end: P(10^9);
MATHEMATICA
Select[Range@ 360, GCD[DivisorSigma[1, #], # (# + 1)/2 - DivisorSigma[1, #]] == 1 &] (* Michael De Vlieger, Aug 27 2015 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Paolo P. Lava, Aug 27 2015
STATUS
approved