A230012 Numbers n such that phi(n) = sigma(n) - reversal(sigma(n)).

19, 199, 437, 603, 1999, 12834, 16348, 21293, 22183, 23383, 25273, 27263, 44377, 46367, 199999, 670661, 691351, 803851, 845321, 1425650, 2103643, 2111191, 2123893, 2174143, 2543773, 2552723, 2753713, 3791659, 4003997, 4034347, 4133332, 4223887, 4244287, 4492429, 4663667

Offset: 1

Author

Farideh Firoozbakht, Feb 04 2014

Keywords

• base
• nonn

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)

Crossrefs

Cf. A000010, A000203, A004086, A069225, A237257.

Programs

Mathematica

r[n_]:=FromDigits[Reverse[IntegerDigits[n]]];Do[If[DivisorSigma[1,n]- r[DivisorSigma[1,n]]==EulerPhi[n],Print[n]],{n,15000000}]