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A060866
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Sum of (d+d') over all unordered pairs (d,d') with d*d' = n.
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7
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2, 3, 4, 9, 6, 12, 8, 15, 16, 18, 12, 28, 14, 24, 24, 35, 18, 39, 20, 42, 32, 36, 24, 60, 36, 42, 40, 56, 30, 72, 32, 63, 48, 54, 48, 97, 38, 60, 56, 90, 42, 96, 44, 84, 78, 72, 48, 124, 64, 93, 72, 98, 54, 120, 72, 120, 80, 90, 60, 168, 62, 96, 104, 135, 84, 144, 68, 126, 96
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OFFSET
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1,1
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COMMENTS
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Paraphrasing the Jovovic formula: if n is not a square then a(n) = sigma(n), the sum of divisors of n, otherwise a(n) = sigma(n) + sqrt(n). - Omar E. Pol, Jun 23 2009
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LINKS
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FORMULA
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a(n) = sum_{i=1..floor(sqrt(n))} (n+i^2)*(1-ceiling(n/i)+floor(n/i))/i. - Wesley Ivan Hurt, Jul 14 2014
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EXAMPLE
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a(4)=9 because pairs of factors are 1*4 and 2*2 and 1+4+2+2=9. a(6)=12 because pairs of factors are 1*6 and 2*3 and 1+6+2+3=12.
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MAPLE
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numtheory[sigma](n) ;
if issqr(n) then
%+sqrt(n) ;
else
% ;
end if;
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MATHEMATICA
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Table[Sum[(i^2 + n) (1 - Ceiling[n/i] + Floor[n/i])/i, {i, Floor[Sqrt[n]]}], {n, 100}] (* Wesley Ivan Hurt, Jul 14 2014 *)
Array[If[IntegerQ@ #2, #3 + #2, #3] & @@ {#, Sqrt@ #, DivisorSigma[1, #]} &, 69] (* Michael De Vlieger, Nov 23 2017 *)
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PROG
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(PARI)
A037213(n) = if(issquare(n, &n), n, 0);
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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