OFFSET
1,1
COMMENTS
If p is prime and p^2 + 1 squarefree, then p^2*(p^2 + 1) is in the sequence.
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..282 (terms < 5*10^11; first 80 terms from Charles R Greathouse IV)
EXAMPLE
13000 is a term because core(13000) = 130 = 100 + 25 + 4 + 1.
PROG
(PARI)
for(n=2, 10^8, if(core(n)==sumdiv(n, d, d*issquare(d)), print(n)))
(PARI) ssd(f)=prod(i=1, #f[, 1], (f[i, 1]^(f[i, 2]+2-f[i, 2]%2)-1)/(f[i, 1]^2-1))
is(n)=my(f=factor(n)); prod(i=1, #f[, 1], f[i, 1]^(f[i, 2]%2))==ssd(f) && n>1 \\ Charles R Greathouse IV, May 20 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Antonio Roldán, May 19 2013
STATUS
approved