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 A046080 a(n) is the number of integer-sided right triangles with hypotenuse n. 53
 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 0, 0, 0, 2, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 0, 2, 1, 1, 1, 0, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 4, 0, 0, 1, 0, 1, 0, 0, 1, 1, 2, 0, 0, 1, 0, 1, 0, 1, 0, 0, 4, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,25 COMMENTS Or number of ways n^2 can be written as the sum of two positive squares: a(5) = 1: 3^2 + 4^2 = 5^2; a(25) = 2: 7^2 + 24^2 = 15^2 + 20^2 = 25^2. - Alois P. Heinz, Aug 01 2019 REFERENCES A. H. Beiler, Recreations in the Theory of Numbers, New York: Dover, pp. 116-117, 1966. LINKS Stanislav Sykora, Table of n, a(n) for n = 1..20000 Ron Knott, Pythagorean Triples and Online Calculators F. Richman, Pythagorean Triples A. Tripathi, On Pythagorean triples containing a fixed integer, Fib. Q., 46/47 (2008/2009), 331-340. See Theorem 7. Eric Weisstein's World of Mathematics, Pythagorean Triple FORMULA Let n = 2^e_2 * product_i p_i^f_i * product_j q_j^g_j where p_i == 1 mod 4, q_j == 3 mod 4; then a(n) = (1/2)*(product_i (2*f_i + 1) - 1). - Beiler, corrected 8*a(n) + 4 = A046109(n) for n > 0. - Ralf Stephan, Mar 14 2004 a(n) = 0 for n in A004144. - Lekraj Beedassy, May 14 2004 a(A084645(k)) = 1. - Ruediger Jehn, Jan 14 2022 a(A084646(k)) = 2. - Ruediger Jehn, Jan 14 2022 a(A084647(k)) = 3. - Jean-Christophe Hervé, Dec 01 2013 a(A084648(k)) = 4. - Jean-Christophe Hervé, Dec 01 2013 a(A084649(k)) = 5. - Jean-Christophe Hervé, Dec 01 2013 a(n) = A063725(n^2) / 2. - Michael Somos, Mar 29 2015 a(n) = Sum_{k=1..n} Sum_{i=1..k} [i^2 + k^2 = n^2], where [ ] is the Iverson bracket. - Wesley Ivan Hurt, Dec 10 2021 a(A002144(k)^n) = n. - Ruediger Jehn, Jan 14 2022 MAPLE f:= proc(n) local F, t; F:= select(t -> t[1] mod 4 = 1, ifactors(n)[2]); 1/2*(mul(2*t[2]+1, t=F)-1) end proc: map(f, [\$1..100]); # Robert Israel, Jul 18 2016 MATHEMATICA a[1] = 0; a[n_] := With[{fi = Select[ FactorInteger[n], Mod[#[[1]], 4] == 1 & ][[All, 2]]}, (Times @@ (2*fi+1)-1)/2]; Table[a[n], {n, 1, 99}] (* Jean-François Alcover, Feb 06 2012, after first formula *) PROG (PARI) a(n)={my(m=0, k=n, n2=n*n, k2, l2); while(1, k=k-1; k2=k*k; l2=n2-k2; if(l2>k2, break); if(issquare(l2), m++)); return(m)} \\ brute force, Stanislav Sykora, Mar 18 2015 (PARI) {a(n) = if( n<1, 0, sum(k=1, sqrtint(n^2 \ 2), issquare(n^2 - k^2)))}; /* Michael Somos, Mar 29 2015 */ (PARI) a(n) = {my(f = factor(n/(2^valuation(n, 2)))); (prod(k=1, #f~, if ((f[k, 1] % 4) == 1, 2*f[k, 2] + 1, 1)) - 1)/2; } \\ Michel Marcus, Mar 08 2016 (Python) from math import prod from sympy import factorint def A046080(n): return prod((e<<1)+1 for p, e in factorint(n).items() if p&3==1)>>1 # Chai Wah Wu, Sep 06 2022 CROSSREFS First differs from A083025 at n=65. Cf. A000290, A006339, A024362, A046079, A046081, A009000. A088111 gives records; A088959 gives where records occur. Cf. A046109, A063725. Cf. A004144, A084647, A084648, A084649. Partial sums: A224921. Sequence in context: A088950 A267113 A083025 * A170967 A035227 A355320 Adjacent sequences: A046077 A046078 A046079 * A046081 A046082 A046083 KEYWORD nonn AUTHOR Eric W. Weisstein STATUS approved

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Last modified June 5 16:50 EDT 2023. Contains 363137 sequences. (Running on oeis4.)