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A102574
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a(n) is the sum of the distinct norms of the divisors of n over the Gaussian integers.
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2
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1, 7, 10, 31, 31, 70, 50, 127, 91, 217, 122, 310, 183, 350, 310, 511, 307, 637, 362, 961, 500, 854, 530, 1270, 781, 1281, 820, 1550, 871, 2170, 962, 2047, 1220, 2149, 1550, 2821, 1407, 2534, 1830, 3937, 1723, 3500, 1850, 3782, 2821, 3710, 2210, 5110, 2451
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OFFSET
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1,2
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COMMENTS
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Also sum of divisors of n^2 which are the sum of two squares (A001481). For example the divisors of 3^2 are 1, 3, 9 of which only 1 and 9 are in A001481 and a(3) = 1 + 9 = 10. - Jianing Song, Aug 03 2018
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LINKS
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FORMULA
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Multiplicative with a(p^e) = sigma(p^(2e)) = (p^(2e+1) - 1)/(p - 1) if p = 2 or p == 1 (mod 4); sigma_2(p^e) = (p^(2e+2) - 1)/(p^2 - 1) if p == 3 (mod 4). - Jianing Song, Aug 03 2018
Sum_{k=1..n} a(k) ~ c * n^3, where c = (5/12) * zeta(3) * A243380 = 0.52812367275583317729... . - Amiram Eldar, Feb 13 2024
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EXAMPLE
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Let ||i|| denote the norm of i.
a(2) = 1 + ||1+i|| + 2^2 = 1 + 2 + 4 = 7.
a(5) = 1 + ||1+2i|| + 5^2 = 1 + 5 + 25 = 31. Note that ||1+2i|| = ||2+i|| so their norm (5) is only counted once.
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MATHEMATICA
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b[n_] := Product[{p, e} = pe; If[Mod[p, 4] == 3, p^e, 1], {pe, FactorInteger[n]}];
a[n_] := With[{r = b[n]}, DivisorSigma[2, r] DivisorSigma[1, (n/r)^2]];
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PROG
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b(n)={my(f=factor(n)); my(r=prod(i=1, #f~, my([p, e]=f[i, ]); if(p%4==3, p^e, 1))); r}
a(n)={my(r=b(n)); sigma(r, 2)*sigma((n/r)^2)} \\ Andrew Howroyd, Aug 03 2018
(Python)
from math import prod
from sympy import factorint
def A102574(n): return prod((q := int(p & 3 == 3))*(p**(2*(e+1))-1)//(p**2-1) + (1-q)*(p**(2*e+1)-1)//(p-1) for p, e in factorint(n).items()) # Chai Wah Wu, Jun 28 2022
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CROSSREFS
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KEYWORD
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nonn,mult,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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