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Search: id:A136543
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| A136543 |
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Numbers n such that phi(n)+sigma(n)=4*reversal(n). |
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1
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OFFSET
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1,1
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COMMENT
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All semiprimes of the form 8*10^m-7 are in the sequence - the proof is easy. Next term is greater than 10^8.
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EXAMPLE
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phi(2152311)+sigma(2152311)=1217664+3312384=4*1132512=4*reversal(2152311), so 2152311 is in the sequence.
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MATHEMATICA
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Do[If[4*FromDigits@Reverse@IntegerDigits@n==EulerPhi@n+ DivisorSigma[1, n], Print[n]], {n, 100000000}]
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CROSSREFS
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Sequence in context: A130555 A133537 A075667 this_sequence A133274 A086393 A108251
Adjacent sequences: A136540 A136541 A136542 this_sequence A136544 A136545 A136546
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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), Jan 20 2008
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