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A224770
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Numbers that are the primitive sum of two squares in exactly two ways.
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6
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65, 85, 130, 145, 170, 185, 205, 221, 265, 290, 305, 325, 365, 370, 377, 410, 425, 442, 445, 481, 485, 493, 505, 530, 533, 545, 565, 610, 629, 650, 685, 689, 697, 725, 730, 745, 754, 785, 793, 845, 850, 865, 890, 901, 905, 925, 949, 962, 965, 970
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OFFSET
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1,1
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COMMENTS
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These are the increasingly ordered numbers a(n) which satisfy A193138(a(n)) = 2.
Neither the order of the squares nor the signs of the numbers to be squared are taken into account. The two squares are necessarily distinct and each is nonzero.
This sequence is a proper subsequence of A000404.
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LINKS
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FORMULA
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a(n) = a^2 + b^2, a and integers, 0 < a < b and gcd(a,b) = 1 in exactly two ways. These representations of a(n) are denoted by two different pairs (a,b).
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EXAMPLE
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n=1, 65: (1, 8), (4, 7),
n=2, 85: (2, 9), (6, 7),
n=3, 130: (3, 11), (7, 9),
n=4, 145: (1, 12), (8, 9),
n=5, 170: (1, 13), (7, 11),
n=6, 185: (4, 13), (8, 11),
n=7, 205: (3, 14), (6, 13),
n=8, 221: (5, 14), (10, 11),
n=9, 265: (3, 16), (11, 12),
n=10, 290: (1, 17), (11, 13).
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MATHEMATICA
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nn = 35; t = Sort[Select[Flatten[Table[If[GCD[a, b] == 1, a^2 + b^2, 0], {a, nn}, {b, a, nn}]], 0 < # <= nn^2 &]]; Transpose[Select[Tally[t], #[[2]] == 2 &]][[1]] (* T. D. Noe, Apr 20 2013 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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