OFFSET
1,1
EXAMPLE
a(1)= 3 = sigma(2);
a(2)= 7 = sigma(sigma(3));
a(3)= 28 = sigma(sigma(sigma(5)));
a(4)= 60 = sigma(sigma(sigma(sigma(7)))).
MAPLE
a:= n-> (numtheory[sigma]@@n)(ithprime(n)):
seq(a(n), n=1..30); # Alois P. Heinz, Oct 03 2018
MATHEMATICA
f[n_] := DivisorSigma[1, n]; Table[ Nest[f, Prime[n], n], {n, 25}] (* Robert G. Wilson v, Dec 22 2004 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jorge Coveiro, Dec 22 2004
EXTENSIONS
Edited and extended by Robert G. Wilson v, Dec 22 2004
STATUS
approved