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A292770
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If sigma(n)+phi(n) is even then (sigma(n)+phi(n))/2 otherwise -1.
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2
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1, 2, 3, -1, 5, 7, 7, -1, -1, 11, 11, 16, 13, 15, 16, -1, 17, -1, 19, 25, 22, 23, 23, 34, -1, 27, 29, 34, 29, 40, 31, -1, 34, 35, 36, -1, 37, 39, 40, 53, 41, 54, 43, 52, 51, 47, 47, 70, -1, -1, 52, 61, 53, 69, 56, 72, 58, 59, 59, 92, 61, 63, 70, -1, 66, 82, 67, 79, 70, 84, 71, -1, 73, 75, 82, 88, 78, 96
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OFFSET
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1,2
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LINKS
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MAPLE
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f:= proc(n) local v;
v:= numtheory:-sigma(n)+numtheory:-phi(n);
if v::even then v/2 else -1 fi
end proc:
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MATHEMATICA
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Table[If[EvenQ[DivisorSigma[1, n]+EulerPhi[n]], (DivisorSigma[1, n]+EulerPhi[n])/2, -1], {n, 120}] (* Harvey P. Dale, Jun 19 2022 *)
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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