A136538 Numbers n such that reversal(n)=2*phi(n).

2, 4, 8, 42, 84, 2763, 4032, 8064, 67314, 86558, 291483, 2700063, 2700000063, 4039603962, 46420566582, 6739054689866

Offset: 1

Author

Farideh Firoozbakht, Jan 04 2008

Keywords

• nonn
• base
• more

Comments

If m>1 and p=3*10^m+7 is prime then n=9*p is in the sequence (the proof is easy). If n is an even term of the sequence and the largest digit of n is less than 5(3) then 2n is (both numbers 2n & 4n are) in the sequence (the proof is easy).

a(17) > 10^13. - Giovanni Resta, Aug 12 2019

Examples

			Reversal(42)=24=2*12=2*phi(42), so 42 is in the sequence. [Example corrected Jan 25 2008]
		

Crossrefs

Cf. A000010, A004086, A069215, A114930, A114931, A136539.

Programs

Mathematica

Do[If[FromDigits@Reverse@IntegerDigits@n==2*EulerPhi[n], Print[n]],{n,100000000}]

Extensions

a(13)-a(15) from Giovanni Resta, Oct 28 2012

a(16) from Giovanni Resta, Aug 12 2019