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A136543
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Numbers n such that phi(n)+sigma(n)=4*reversal(n).
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2
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OFFSET
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1,1
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COMMENTS
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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.
a(7) > 10^12. - Giovanni Resta, Nov 03 2012
A100412(n) is in the sequence for n = 2, 4, 7, 9, 11, 16, 18, 23, 31, 32, 40,... - M. F. Hasler, Nov 03 2012
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LINKS
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Table of n, a(n) for n=1..6.
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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: A133537 A213471 A075667 * A133274 A261657 A086393
Adjacent sequences: A136540 A136541 A136542 * 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, Jan 20 2008
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EXTENSIONS
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a(5)-a(6) from Giovanni Resta, Nov 03 2012
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STATUS
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approved
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