OFFSET
1,2
COMMENTS
sigma(a(6)) >= 10^12. - Hiroaki Yamanouchi, Sep 26 2014
EXAMPLE
sigma(a(2)) = (64*9*5*2)^2 and sigma(a(2)^2) = (3*7*13*19*31*127)^2.
sigma(a(3)) = (64*9*5*7)^2 and sigma(a(3)^2) = (3*7*13*37*61*499)^2.
sigma(a(4)) = (256*9*5*47)^2 and sigma(a(4)^2) = (3*49*13*19*37*43*61*67)^2.
sigma(a(5)) = (16*3*7*121*17)^2 and sigma(a(5)^2) = (3*49*13*31*61*109*757)^2.
PROG
(PARI) isok(n) = issquare(sigma(n)) && issquare(sigma(n^2)); \\ Michel Marcus, Sep 24 2014
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Alex Ratushnyak, Nov 24 2013
EXTENSIONS
a(4) from Hiroaki Yamanouchi, Sep 24 2014
a(5) from Hiroaki Yamanouchi, Sep 26 2014
STATUS
approved