A230019 Numbers n such that sigma(n) + phi(n) = reversal(n) - 3.

199993, 17333334, 19999993, 199999999993, 1999999999993

Offset: 1

Author

Farideh Firoozbakht, Dec 02 2013

Keywords

• nonn
• base
• more

Comments

If n=2*10^m-7 is a semiprime then n is in the sequence. Also if p=(1/999)*(962*1000^m+37) is prime then 18*p is in the sequence. All known terms are of these two forms.

a(6) > 10^13. - Giovanni Resta, Feb 08 2014

Crossrefs

Cf. A000010, A000203, A004086, A070272, A230004, A230005, A230006, A136544, A237521, A237522.

Programs

Mathematica

r[n_] := FromDigits[Reverse[IntegerDigits[n]]]; Do[If[DivisorSigma[1, n] + EulerPhi[n] == r[n]-3, Print[n]], {n, 1000000000}]

Extensions

a(4)-a(5) from Giovanni Resta, Feb 06 2014