OFFSET
1,2
COMMENTS
Solutions to A076360(x) = 0.
Assuming Schinzel's hypothesis is true, the sequence is infinite. That conjecture implies that there are infinitely many primes p for which (p^2 + p + 1)/3 is prime. (E.g., p = 7, 13, 19, 31, 43, 73, 97, ...) For such p, we have d(sigma(p^2)) = d(p^2+p+1) = 4 and sigma(d(p^2)) = sigma(3) = 4, so p^2 is in the sequence. - Dean Hickerson, Jan 24 2006
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10029
Eric Weisstein's World of Mathematics, Schinzel's hypothesis.
MATHEMATICA
d0[x_] := DivisorSigma[0, x] d1[x_] := DivisorSigma[1, x] Do[s=d0[d1[n]]-d1[d0[n]]; If[s==0, Print[n]], {n, 1, 10000}]
Select[Range[1380], DivisorSigma[0, DivisorSigma[1, #]] == DivisorSigma[1, DivisorSigma[0, #]] &] (* Jayanta Basu, Mar 26 2013 *)
PROG
(PARI) is(n)=numdiv(sigma(n))==sigma(numdiv(n)) \\ Charles R Greathouse IV, Jun 25 2013
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Labos Elemer, Oct 08 2002
STATUS
approved