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 A020898 Positive cubefree integers n such that the Diophantine equation X^3 + Y^3 = n*Z^3 has solutions. 11
 2, 6, 7, 9, 12, 13, 15, 17, 19, 20, 22, 26, 28, 30, 31, 33, 34, 35, 37, 42, 43, 49, 50, 51, 53, 58, 61, 62, 63, 65, 67, 68, 69, 70, 71, 75, 78, 79, 84, 85, 86, 87, 89, 90, 91, 92, 94, 97, 98, 103, 105, 106, 107, 110, 114, 115, 117, 123, 124, 126, 127, 130 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS These numbers are the cubefree sums of two nonzero rational cubes. This sequence does not contain A202679, which has members that are not cubefree. - Robert Israel, Mar 16 2016 Notice that 34^3 + 74^3 = 48*21^3 = 6*42^3 because 48 = 6*2^3 is not cubefree, but now 17^3 + 37^3 = 6*21^3 and 6 is already listed in the sequence. - Michael Somos, Mar 13 2023 REFERENCES B. N. Delone and D. K. Faddeev, The Theory of Irrationalities of the Third Degree, Amer. Math. Soc., 1964. L. E. Dickson, History of The Theory of Numbers, Vol. 2, Chap. XXI, Chelsea NY 1966. L. J. Mordell, Diophantine Equations, Academic Press, Chap. 15. LINKS David W. Wilson, Table of n, a(n) for n = 1..255 (from Finch paper) J. H. E. Cohn, The £ 450 question, Math. Mag., 73 (no. 3, 2000), 220-226. Steven R. Finch, On a Generalized Fermat-Wiles Equation [broken link] Steven R. Finch, On a Generalized Fermat-Wiles Equation [From the Wayback Machine] EXAMPLE 37^3 + 17^3 = 6*21^3 is the smallest positive solution for n = 6 (found by Lagrange). 5^3 + 4^3 = 7*3^3 is the smallest positive solution for n = 7. MATHEMATICA (* A naive program with a few pre-computed terms *) nmax = 130; xmax = 2000; CubeFreePart[n_] := Times @@ Power @@@ ({#[[1]], Mod[#[[2]], 3]} & /@ FactorInteger[n]); nn = Reap[ Do[ n = CubeFreePart[ x*y*(x+y) ]; If[ 1 < n <= nmax, Sow[n]], {x, 1, xmax}, {y, x, xmax}]][[2, 1]] // Union; A020898 = Union[nn, {17, 31, 53, 67, 71, 79, 89, 94, 97, 103, 107, 123}](* Jean-François Alcover, Mar 30 2012 *) CROSSREFS Cf. A159843, A166246, A228499, A254324, A254326. Sequence in context: A061416 A190247 A020897 * A184779 A200926 A047277 Adjacent sequences: A020895 A020896 A020897 * A020899 A020900 A020901 KEYWORD nonn,nice,changed AUTHOR EXTENSIONS Entry revised by N. J. A. Sloane, Aug 12 2004 Links updated by Max Alekseyev, Oct 17 2007 and Dec 12 2007 STATUS approved

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Last modified March 25 04:11 EDT 2023. Contains 361511 sequences. (Running on oeis4.)