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 A230561 Smallest number that is the sum of two positive n-th powers in >= n ways. 5
 2, 50, 87539319 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Guy, 2004: "Euler knew that 635318657 = 133^4 + 134^4 = 59^4 + 158^4, and Leech showed this to be the smallest example. No one knows of three such equal sums." Thus no one knows whether a(4) exists, which requires four such equal sums. a(4) > 10^21 (if it exists). There is no number <= 10^21 that is the sum of two positive 4th powers in >= three ways. - Donovan Johnson, Jan 07 2014 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, 3rd edition, Springer, 2004, D1. LINKS FORMULA a(n) >= A016078(n) for n > 1, with equality at least for n = 2, and inequality at least for n = 3. EXAMPLE 2 = 1^1 + 1^1. 50 = 1^2 + 7^2 = 5^2 + 5^2. 87539319 = 167^3 + 436^3 = 228^3 + 423^3 = 255^3 + 414^3. CROSSREFS Cf. A048610, A011541 for a(2), a(3). Cf. also A016078, A230477. Sequence in context: A080293 A084107 A203746 * A057943 A037419 A078165 Adjacent sequences:  A230558 A230559 A230560 * A230562 A230563 A230564 KEYWORD hard,more,nonn,bref AUTHOR Jonathan Sondow, Oct 23 2013 STATUS approved

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Last modified June 1 07:40 EDT 2020. Contains 334759 sequences. (Running on oeis4.)