This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A257408 #11 Apr 22 2015 11:13:18
%S A257408 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,
%T A257408 67,68,71,73,76,79,83,89,92,97,101,103,107,109,113,116,124,127,131,
%U A257408 137,139,148,149,151,157,163,164,167,172,173,179,181,188,191,193
%N A257408 Values of n such that there is exactly 1 solution to x^2 - y^2 = n with x > y >= 0.
%C A257408 The terms a(1)=1 and a(3)=4 are the only squares in this sequence. - _M. F. Hasler_, Apr 22 2015
%H A257408 Colin Barker, <a href="/A257408/b257408.txt">Table of n, a(n) for n = 1..1500</a>
%e A257408 13 is in the sequence because there is only 1 solution to x^2 - y^2 = 13, namely (x,y) = (7,6).
%t A257408 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 *)
%o A257408 (PARI) is(n)=A034178(n)==1 \\ _M. F. Hasler_, Apr 22 2015
%Y A257408 Cf. A257409-A257417.
%Y A257408 Cf. A034178.
%K A257408 nonn
%O A257408 1,2
%A A257408 _Colin Barker_, Apr 22 2015