A047697 Smallest positive number that can be written in n ways as a sum of two (not necessarily positive) coprime cubes.

1, 91, 3367, 16776487, 506433677359393

Offset

1,2

References

R. K. Guy, Unsolved Problems in Number Theory, Section D1.

Links

Table of n, a(n) for n=1..5.

D. J. Bernstein, Enumerating solutions to p(a) + q(b) = r(c) + s(d)

Mathematica

(* This naive program is not convenient for more than 4 terms *) max = 300; Clear[k]; k[_] = 0; t = Reap[ Do[ Do[z = x^3 + y^3; If[CoprimeQ[x, y], k[z] = k[z] + 1]; z = -x^3 + y^3; If[CoprimeQ[x, y], k[z] = k[z] + 1]; kz = k[z]; If[kz > 1, Sow[{z, kz}]], {y, x, max}], {x, 1, max}]][[2, 1]]; s = Sort[t]; a[n_] := Select[s, #[[2]] == n &, 1][[1, 1]]; a[1] = 1; Table[a[n], {n, 1, 4}] (* Jean-François Alcover, Jan 11 2013 *)

Crossrefs

Cf. A047696.

Sequence in context: A133416 A169937 A378965 * A096054 A129965 A017807

Adjacent sequences: A047694 A047695 A047696 * A047698 A047699 A047700

Keyword

nonn,nice,hard

Author

N. J. A. Sloane

Status

approved