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A047697 Smallest positive number that can be written in n ways as a sum of two (not necessarily positive) coprime cubes. 1
1, 91, 3367, 16776487, 506433677359393 (list; graph; refs; listen; history; text; internal format)
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-Fran├žois Alcover, Jan 11 2013 *)

CROSSREFS

Cf. A047696.

Sequence in context: A221738 A133416 A169937 * A096054 A129965 A017807

Adjacent sequences:  A047694 A047695 A047696 * A047698 A047699 A047700

KEYWORD

nonn,nice,hard

AUTHOR

N. J. A. Sloane

STATUS

approved

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Last modified December 10 20:55 EST 2018. Contains 318049 sequences. (Running on oeis4.)