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 A301858 Positive integers which can be written as the sum of two squares but cannot be written as x^2 + y^2 + 2*z^2 with x and y integers and z a nonzero integer. 1
 1, 5, 29, 65 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The sequence has no term in the interval [66, 10^6]. Conjecture 1: The sequence only has the four terms 1, 5, 29 and 65. Conjecture 2: For any integer n > 1 which is neither 17 nor a power of 2, if n = u^2 + 2*v^2 for some integers u and v, then n = x^2 + 2*y^2 + 3*z^2 for some integers x,y,z with z nonzero. Conjecture 3: For any positive integer n not of the form 4^k*m (k = 0,1,2,... and m = 1, 7, 13), if n = u^2 + 3*v^2 for some integers u and v, then n = x^2 + 2*y^2 + 3*z^2 for some integers x,y,z with y nonzero. LINKS Zhi-Wei Sun, Table of n, a(n) for n = 1..4 MATHEMATICA 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]); SQ[n_]:=SQ[n]=IntegerQ[Sqrt[n]]; tab={}; Do[If[QQ[m]==False, Goto[aa]]; Do[If[SQ[m-2x^2-y^2], Goto[aa]], {x, 1, Sqrt[m/2]}, {y, 0, Sqrt[(m-2x^2)/2]}]; tab=Append[tab, m]; Label[aa], {m, 1, 1000}]; Print[tab] CROSSREFS Cf. A000290, A000549, A001481, A002479, A301471. Sequence in context: A115706 A031394 A103094 * A293174 A108928 A097812 Adjacent sequences:  A301855 A301856 A301857 * A301859 A301860 A301861 KEYWORD nonn AUTHOR Zhi-Wei Sun, Mar 27 2018 STATUS approved

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Last modified August 11 06:25 EDT 2020. Contains 336422 sequences. (Running on oeis4.)