A114080 - OEIS

%I #9 Jul 22 2021 02:01:04

%S 2,4,9,16,25,64,729,65536

%N Numbers k such that sigma(k) times the k-th prime is a brilliant number (A078972).

%C No additional terms up to 1 million. - _Harvey P. Dale_, Nov 13 2013

%e sigma(25) * p(25) = 3007 = 31*97.

%t brilQ[n_]:=Module[{pf=Transpose[FactorInteger[n]][[1]]},PrimeOmega[n] == 2&&IntegerLength[First[pf]]==IntegerLength[Last[pf]]]; Select[Range[ 100000], brilQ[DivisorSigma[1,#]Prime[#]]&] (* _Harvey P. Dale_, Nov 13 2013 *)

%K nonn,base

%O 1,1

%A _Giovanni Resta_, Feb 13 2006