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A201880
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Numbers n such that sigma_2(n) - n^2 is prime.
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1
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4, 18, 21, 33, 39, 72, 93, 99, 100, 159, 171, 177, 189, 207, 213, 231, 245, 249, 261, 275, 291, 297, 303, 333, 338, 357, 369, 381, 399, 400, 453, 471, 475, 477, 484, 495, 537, 539, 543, 561, 609, 633, 648, 657, 669, 681, 711, 717, 783, 801, 833, 861, 909, 927
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listen;
history;
text;
internal format)
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OFFSET
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1,1
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COMMENTS
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Numbers n such that sum of the squares of the proper (or aliquot) divisors of n is a prime number.
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LINKS
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FORMULA
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EXAMPLE
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a(3)=21 because the aliquot divisors of 21 are 1, 3, 7, the sum of whose squares is 1^2 + 3^2 + 7^2 = 59, prime.
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MAPLE
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numtheory[sigma][2](n)-n^2 ;
end proc:
isA201880 := proc(n)
end proc:
for n from 1 to 1000 do
if isA201880(n) then
printf("%d, ", n);
end if;
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MATHEMATICA
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Select[Range[400], PrimeQ[DivisorSigma[2, #]-#^2]&]
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PROG
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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