OFFSET
1,1
COMMENTS
A prime p is in the sequence iff p is of the form 2*10^m-1. 19, 199, 1999 are such terms of the sequence.
If both numbers p=2*10^m-1 and q=2*10^m+3 are prime then n=p*q is in the sequence. For m=1, 3, 5, 7 both p and q are prime.
If both numbers p=2*10^m-1 and q=(7*10^m-1)/3 are prime then n=p*q is in the sequence. For m=1, 2, 3, 53 both p and q are prime.
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..103 (terms < 10^11)
MATHEMATICA
r[n_]:=FromDigits[Reverse[IntegerDigits[n]]]; Do[If[DivisorSigma[1, n]- r[DivisorSigma[1, n]]==EulerPhi[n], Print[n]], {n, 15000000}]
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Farideh Firoozbakht, Feb 04 2014
STATUS
approved