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Search: id:A135238
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| A135238 |
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Numbers n such that phi(sigma(n))=reversal(n). |
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+0
2
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OFFSET
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1,2
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COMMENT
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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.
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EXAMPLE
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phi(sigma(880374))=phi(1920960)=473088=reversal(880374), so 880374 is in the sequence.
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MATHEMATICA
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reversal[n_]:=FromDigits[Reverse[IntegerDigits[n]]]; Do[If[EulerPhi[DivisorSigma[1, n]]==reversal[n], Print[n]], {n, 350000000}]
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CROSSREFS
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Cf. A071525.
Sequence in context: A027733 A054874 A057841 this_sequence A133376 A160814 A038582
Adjacent sequences: A135235 A135236 A135237 this_sequence A135239 A135240 A135241
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KEYWORD
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base,more,nonn
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AUTHOR
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Farideh Firoozbakht (mymontain(AT)yahoo.com), Dec 26 2007
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