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 A335558 Positive integers that cannot be expressed as the sum of at most 5 pairwise coprime squares. 0

%I

%S 21,22,23,24,33,45,46,47,48,57,69,70,71,72,81,93,94,95,96,105,117,118,

%T 119,120,129,141,142,143,144,153,154,161,165,166,167,168,177,189,190,

%U 191,192,201,209,213,214,215,216,217,225,237,238,239,240,246,249,261

%N Positive integers that cannot be expressed as the sum of at most 5 pairwise coprime squares.

%D R. K. Guy, Unsolved Problems in Number Theory, C20.

%t n = 261;

%t a1 = Prime[Range]^2; a2 = a3 = a4 = a5 = {};

%t Do[If[GCD[x, y] == 1, AppendTo[a2, x^2 + y^2]], {x, 0, (n/2)^(1/2)}, {y, x, (n - x^2)^(1/2)}];

%t Do[If[GCD[x, y] == GCD[x, z] == GCD[y, z] == 1, AppendTo[a3, x^2 + y^2 + z^2]], {x, 0, (n/3)^(1/2)}, {y, x, ((n - x^2)/2)^(1/2)}, {z, y, (n - x^2 - y^2)^(1/2)}];

%t Do[If[GCD[x, y] == GCD[x, z] == GCD[x, t] == GCD[y, z] == GCD[y, t] == GCD[z, t] == 1, AppendTo[a4, x^2 + y^2 + z^2 + t^2]], {x, 0, (n/4)^(1/2)}, {y, x, ((n - x^2)/3)^(1/2)}, {z, y, ((n - x^2 - y^2)/2)^(1/2)}, {t, z, (n - x^2 - y^2 - z^2)^(1/2)}];

%t Do[If[GCD[x, y] == GCD[x, z] == GCD[x, t] == GCD[x, w] == GCD[y, z] == GCD[y, t] == GCD[y, w] == GCD[z, t] == GCD[z, w] == GCD[t, w] == 1, AppendTo[a5, x^2 + y^2 + z^2 + t^2 + w^2]], {x, 0, (n/5)^(1/2)}, {y, x, ((n - x^2)/4)^(1/2)}, {z, y, ((n - x^2 - y^2)/3)^(1/2)}, {t, z, ((n - x^2 - y^2 - z^2)/2)^(1/2)}, {w, t, (n - x^2 - y^2 - z^2 - t^2)^(1/2)}];

%t Complement[Range[n], Union@Join[a1, a2, a3, a4, a5]]

%Y Cf. A008784, A280991.

%K nonn

%O 1,1

%A _XU Pingya_, Jun 14 2020

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Last modified May 20 03:37 EDT 2022. Contains 353851 sequences. (Running on oeis4.)