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 A257408 Values of n such that there is exactly 1 solution to x^2 - y^2 = n with x > y >= 0. 11
 1, 3, 4, 5, 7, 8, 11, 12, 13, 17, 19, 20, 23, 28, 29, 31, 37, 41, 43, 44, 47, 52, 53, 59, 61, 67, 68, 71, 73, 76, 79, 83, 89, 92, 97, 101, 103, 107, 109, 113, 116, 124, 127, 131, 137, 139, 148, 149, 151, 157, 163, 164, 167, 172, 173, 179, 181, 188, 191, 193 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The terms a(1)=1 and a(3)=4 are the only squares in this sequence. - M. F. Hasler, Apr 22 2015 LINKS Colin Barker, Table of n, a(n) for n = 1..1500 EXAMPLE 13 is in the sequence because there is only 1 solution to x^2 - y^2 = 13, namely (x,y) = (7,6). MATHEMATICA r[n_] := Reduce[x^2 - y^2 == n && x > y >= 0, {x, y}, Integers]; Reap[For[n = 1, n < 200, n++, If[r[n][[0]] === And, Print[n]; Sow[n]]]][[2, 1]] (* Jean-François Alcover, Apr 22 2015 *) PROG (PARI) is(n)=A034178(n)==1 \\ M. F. Hasler, Apr 22 2015 CROSSREFS Cf. A257409-A257417. Cf. A034178. Sequence in context: A173001 A325130 A047500 * A131613 A138494 A285531 Adjacent sequences:  A257405 A257406 A257407 * A257409 A257410 A257411 KEYWORD nonn AUTHOR Colin Barker, Apr 22 2015 STATUS approved

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Last modified December 14 17:32 EST 2019. Contains 329979 sequences. (Running on oeis4.)