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A267477
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Integers n such that n^2 = (x^3 + y^3) / 2 where x, y > 0, is soluble.
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3
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1, 6, 8, 27, 42, 48, 64, 78, 125, 147, 162, 196, 216, 336, 343, 384, 456, 512, 624, 722, 729, 750, 1000, 1050, 1134, 1176, 1296, 1331, 1342, 1568, 1573, 1674, 1694, 1728, 2028, 2058, 2106, 2197, 2366, 2387, 2450, 2522, 2646, 2688, 2744, 2899, 3072, 3087, 3211, 3375, 3648, 3698
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OFFSET
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1,2
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COMMENTS
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Motivation was the simple question: What are the squares that are the averages of two positive cubes?
Corresponding squares are 1, 36, 64, 729, 1764, 2304, 4096, 6084, 15625, 21609, 26244, 38416, 46656, 112896, 117649, 147456, 207936, 262144, 389376, 521284, ...
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LINKS
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EXAMPLE
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42 is a term because 42^2 = (11^3 + 13^3) / 2.
78 is a term because 78^2 = (1^3 + 23^3) / 2.
147 is a term because 147^2 = (7^3 + 35^3) / 2.
1573 is a term because 1573^2 = (77^3 + 165^3) / 2.
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MATHEMATICA
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Select[Range@1000, Resolve@ Exists[{x, y}, And[Reduce[#^2 == (x^3 + y^3)/2, {x, y}, Integers], x > 0, y > 0]] &] (* Michael De Vlieger, Jan 16 2016 *)
(* or, much faster: *) Select[Range@ 1000, {} != PowersRepresentations[#^2 2, 2, 3] &] (* Giovanni Resta, Nov 26 2018 *)
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PROG
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(PARI) T = thueinit('z^3+1);
is(n) = #select(v->min(v[1], v[2])>0, thue(T, n))>0;
for(n=1, 1e4, if(is(2*n^2), print1(n, ", ")));
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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