OFFSET
1,2
EXAMPLE
sigma(3199808) = 6784086 = triangular(3683), sigma(3199808*2) = 13621590 = triangular(5219), so 3199808 is in the sequence.
PROG
(PARI) is(n)=issquare(8*sigma(n)+1)&&issquare(8*sigma(2*n)+1) \\ Ralf Stephan, Nov 30 2013
CROSSREFS
KEYWORD
nonn,hard
AUTHOR
Alex Ratushnyak, Nov 27 2013
EXTENSIONS
a(9) from Hiroaki Yamanouchi, Oct 02 2014
Terms a(10) onward from Max Alekseyev, Feb 21 2024
STATUS
approved