OFFSET
1,1
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..1000
EXAMPLE
199 is in the sequence because 119=7*17 (the product of two distinct primes) and sigma(119)=8*18=144=12^2 (a square number).
MATHEMATICA
f[n_] := Block[{pf = FactorInteger@ n}, Max @@ Last /@ pf == 1 && Length@ pf == 2]; Select[Range@ 6000, IntegerQ@ Sqrt@ DivisorSigma[1, #] && f@ # &] (* Michael De Vlieger, Mar 17 2015 *)
PROG
(PARI) {for(i=1, 10^4, if(omega(i)==2&&issquarefree(i)&&issquare(sigma(i)), print1(i, ", ")))}
(Haskell)
a256152 n = a256152_list !! (n-1)
256152_list = filter f a006881_list where
f x = a010052' ((spf + 1) * (x `div` spf + 1)) == 1
where spf = a020639 x
-- Reinhard Zumkeller, Apr 06 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Antonio Roldán, Mar 16 2015
STATUS
approved