A136544 Numbers n such that phi(n)+sigma(n)=reversal(n)+3.

3, 3997, 3999997, 168632373, 399999999997, 3999999999997

Offset: 1

Author

Farideh Firoozbakht, Jan 20 2008

Keywords

• base
• more
• nonn

Comments

Next term of the sequence is greater than 10^9. All semiprimes of the form 4*10^m-3 are in the sequence - the proof is easy. For m=3,6,11,12,13,15,16,18,19,24,38,56,60,... 4*10^m-3 is semiprime. Is it true that 3 is the only prime term in the sequence?

a(7) > 10^13. - Giovanni Resta, Feb 08 2014

Examples

			phi(168632373)+sigma(168632373)=87744384+285492480=373236861+3= reversal(168632373)+3, so 168632373 is in the sequence.
		

Crossrefs

Cf. A070272, A230004, A230005, A230006, A230019, A237521, A237522.

Programs

Mathematica

Do[If[DivisorSigma[1,n]+EulerPhi@n==FromDigits@Reverse@IntegerDigits@n+ 3,Print[n]],{n,500000000}]

Extensions

a(5) from Giovanni Resta, Nov 03 2012

a(6) from Giovanni Resta, Feb 06 2014