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 A303539 Number of ordered pairs (k, m) with 0 <= k <= m such that n - binomial(2*k,k) - binomial(2*m,m) can be written as the sum of two squares. 22
 0, 1, 2, 3, 2, 2, 3, 4, 3, 2, 3, 6, 4, 2, 2, 4, 4, 2, 2, 5, 5, 5, 4, 4, 4, 4, 4, 5, 4, 5, 4, 4, 3, 4, 3, 4, 4, 5, 6, 5, 5, 5, 4, 7, 3, 3, 4, 6, 4, 2, 3, 5, 6, 3, 4, 5, 6, 5, 2, 5, 4, 5, 3, 2, 4, 5, 4, 3, 3, 3, 6, 7, 5, 5, 6, 10, 6, 3, 4, 8 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Conjecture: a(n) > 0 for all n > 1. a(n) > 0 for all n = 2..10^10. See also A303540 and A303541 for related sequences. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..10000 Zhi-Wei Sun, Refining Lagrange's four-square theorem, J. Number Theory 175(2017), 167-190. Zhi-Wei Sun, New conjectures on representations of integers (I), Nanjing Univ. J. Math. Biquarterly 34(2017), no. 2, 97-120. Zhi-Wei Sun, Restricted sums of four squares, arXiv:1701.05868 [math.NT], 2017-2018. EXAMPLE a(2) = 1 with 2 - binomial(2*0,0) - binomial(2*0,0) = 0^2 + 0^2. a(3) = 2 with 3 - binomial(2*0,0) - binomial(2*0,0) = 0^2 + 1^2 and 3 - binomial(2*0,0) - binomial(2*1,1) = 0^2 + 0^2. a(5) = 2 with 5 - binomial(2*0,0) - binomial(2*1,1) = 1^2 + 1^2 and 5 - binomial(2*1,1) - binomial(2*1,1) = 0^2 + 1^2. MATHEMATICA c[n_]:=c[n]=Binomial[2n, n]； f[n_]:=f[n]=FactorInteger[n]; g[n_]:=g[n]=Sum[Boole[Mod[Part[Part[f[n], i], 1], 4]==3&&Mod[Part[Part[f[n], i], 2], 2]==1], {i, 1, Length[f[n]]}]==0; QQ[n_]:=QQ[n]=(n==0)||(n>0&&g[n]); tab={}; Do[r=0; k=0; Label[bb]; If[c[k]>n, Goto[aa]]; Do[If[QQ[n-c[k]-c[j]], r=r+1], {j, 0, k}]; k=k+1; Goto[bb]; Label[aa]; tab=Append[tab, r], {n, 1, 80}]; Print[tab] CROSSREFS Cf. A000290, A000984, A001481, A303233, A303234, A303338, A303363, A303389, A303393, A303399, A303428, A303401, A303432, A303434, A303540, A303541, A303543. Sequence in context: A118480 A104377 A109337 * A303540 A137266 A062948 Adjacent sequences:  A303536 A303537 A303538 * A303540 A303541 A303542 KEYWORD nonn AUTHOR Zhi-Wei Sun, Apr 25 2018 STATUS approved

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Last modified April 5 22:44 EDT 2020. Contains 333260 sequences. (Running on oeis4.)