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 A046881 Smallest number that is sum of 2 positive distinct n-th powers in 2 different ways. 3
 5, 65, 1729, 635318657 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Randy Ekl discovered that a number that can be written in two ways as a sum of two fifth powers exceeds 2^74 and one that can be written in two ways as a sum of two sixth powers exceeds 2^89. - Jonathan Vos Post, Nov 28 2007 According to the Mathworld links below, a(5) and a(6), if they exist, exceed 1.02*10^26 and 7.25*10^26, respectively. The page at the SquaresOfCubes link below says Stuart Gascoigne did an exhaustive search and found in Sep 2002 that no a(5) solution less than 3.26*10^32 exists. My exhaustive search has determined that any solutions for n > 5, if they exist, must exceed 2^96 (about 7.92*10^28). - Jon E. Schoenfield, Dec 15 2008 REFERENCES R. Alter, Computations and generalizations on a remark of Ramanujan, pp. 182-196 of "Analytic Number Theory (Philadelphia, 1980)", ed. M. I. Knopp, Lect. Notes Math., Vol. 899, 1981. LINKS Christian Boyer, Squares of Cubes. Weisstein, Eric W., Diophantine Equation--5th Powers Weisstein, Eric W., Diophantine Equation--6th Powers Tom Womack, Equal Sums of Like Powers. EXAMPLE 5 = 1^1 + 4^1 = 2^1 + 3^1; 65 = 1^2 + 8^2 = 4^2 + 7^2; 1729 = 1^3 + 12^3 = 9^3 + 10^3; etc. MATHEMATICA (* This naive program is not convenient for n > 3 *) r[n_, k_] := Reduce[0 < x < y && x^n + y^n == k, {x, y}, Integers]; a[n_] := Catch[ For[ k = 1, True, k++, rk = r[n, k]; If[rk =!= False, If[ Head[rk] == Or && Length[rk] == 2, Print["n = ", n, ", k = ", k]; Throw[k]]]]]; Table[a[n], {n, 1, 3}] (* Jean-François Alcover, Jul 30 2013 *) CROSSREFS Cf. A016078. Sequence in context: A277347 A276755 A218221 * A300489 A214348 A195196 Adjacent sequences:  A046878 A046879 A046880 * A046882 A046883 A046884 KEYWORD nonn,nice,hard AUTHOR STATUS approved

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Last modified February 21 06:40 EST 2019. Contains 320371 sequences. (Running on oeis4.)