OFFSET
1,1
COMMENTS
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
FORMULA
d(d(d(a(n)))) = 2 for all n.
A036459(a(n)) = 3. - Ivan Neretin, Jan 25 2016
EXAMPLE
a(15) = 39 and d(39) = 4, d(d(39)) = d(4) = 3 and d(d(d(39))) = 2. After 3 iteration the equilibrium is reached.
MAPLE
filter:= proc(n) local r;
r:= numtheory:-tau(numtheory:-tau(n));
r::odd and isprime(r)
end proc:
select(filter, [$1..1000]); # Robert Israel, Feb 02 2016
MATHEMATICA
fQ[n_] := Module[{d2 = DivisorSigma[0, DivisorSigma[0, n]]}, d2 > 2 && PrimeQ[d2]]; Select[Range[200], fQ] (* T. D. Noe, Jan 22 2013 *)
PROG
(PARI) is(n)=isprime(n=numdiv(numdiv(n))) && n>2 \\ Charles R Greathouse IV, Jan 22 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
Definition clarified by R. J. Mathar and Charles R Greathouse IV, Jan 22 2013
STATUS
approved