OFFSET
1,2
COMMENTS
Contains exactly odd terms of A008848.
Odd n is a term if and only if both n and sigma(n) are perfect squares.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..125
Vladimir Letsko Mathematical Marathon, problem 180 (in Russian)
EXAMPLE
81 is a term because 81, sigma(81) = 121 and sigma(sigma(81)) = 133 are all odd.
PROG
(PARI) forstep(r=1, 5147241, 2, n=r^2; if(sigma(sigma(n))%2<>0, print1(n ", "))) \\ Donovan Johnson, Nov 09 2013
(PARI) forstep(r=1, 5147241, 2, if(!isprime(r)&&issquare(sigma(n=r^2)), print1(n", "))) \\ Charles R Greathouse IV, Nov 12 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Letsko, Nov 09 2013
EXTENSIONS
a(5) corrected by Donovan Johnson, Nov 09 2013
STATUS
approved