OFFSET
1,2
COMMENTS
If both numbers 10^m-3 & 5*10^(m-1)-1 are primes and n=3*(10^m-3) then phi(sigma(n))=reversal(n), namely n is in the sequence (the proof is easy). Conjecture: n=2991 is the only such term of the sequence. there is no further term up to 35*10^7.
There are no other terms up to 10^10. - Donovan Johnson, Oct 24 2013
If p and 2*p-1 are primes, where p = 3900000*100^t + 108900*10^t + 109, then 6*p-3 is in the sequence. This happens at least for t=1 (2346534651), t=2 (234065340651), t=11, and t=76. - Giovanni Resta, Aug 09 2019
EXAMPLE
phi(sigma(880374)) = phi(1920960) = 473088 = reversal(880374), so 880374 is in the sequence.
MATHEMATICA
reversal[n_]:=FromDigits[Reverse[IntegerDigits[n]]]; Do[If[EulerPhi[DivisorSigma[1, n]]==reversal[n], Print[n]], {n, 350000000}]
PROG
(PARI) isok(n) = eulerphi(sigma(n)) == fromdigits(Vecrev(digits(n))); \\ Michel Marcus, Aug 09 2019
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Farideh Firoozbakht, Dec 26 2007
EXTENSIONS
a(7) from Donovan Johnson, Oct 24 2013
a(8)-a(9) from Giovanni Resta, Aug 09 2019
STATUS
approved