OFFSET
1,1
COMMENTS
The equation x^3 + y^3 + z^3 = 0 is solvable in numbers of the form N + M*sqrt(-a(n)), where M and N are integers. Moreover, it is solvable in numbers of the form N + M*sqrt(-l), where l>0 has the form l = A007913(4*k*m^3 - k^4), where k,m >= 1 (without restrictions k,m <= 5*l). But in this more general case there could be unknown numbers l having this form; this circumstance does not allow construction of the full sequence of such l. Therefore we restrict ourselves by the condition k,m <= 5*l. Note that testing l with respect to this condition is rather simple by sorting all values of k,m <= l. One can prove that, at least, in case the Fermat numbers (A000215) are squarefree, the sequence is infinite. Conjecture (necessity of the form of l): If the equation x^3 + y^3 + z^3 = 0 is solvable in numbers of the form N + M*sqrt(-l) with integer N,M, then there exist positive integers k,m such that l = A007913(4*k*m^3 - k^4).
FORMULA
EXAMPLE
MATHEMATICA
squareFreePart[n_] := Times @@ (#[[1]] ^ Mod[ #[[2]], 2] & /@ FactorInteger@n);
fQ[n_] := If[b = 0; SquareFreeQ@n, Block[{k = 1, m}, While[k < 5 n + 1, m = -5 n; While[m < 5 n + 1, a = 4 k*m^3 - k^4; If[a > 0, a = squareFreePart@ a, a = 0]; If[a == n, b = a; Print[{a, k, m}]; Goto@ fini, 0]; m++ ]; k++ ]]; Label@ fini; b == n, False];
k = 1; lst = {}; While[k < 300, If[ fQ@k, AppendTo[lst, k]]; k++ ]; lst
(* Robert G. Wilson v, Aug 29 2010 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Shevelev, Aug 28 2010, Aug 29 2010
EXTENSIONS
Missing term 35 added and a(14)-a(30) from Robert G. Wilson v, Aug 29 2010
a(31)-a(63) from Robert G. Wilson v, Sep 04 2010
STATUS
approved