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%I #18 Jul 15 2018 08:03:00
%S 9,15,16,21,24,25,27,32,33,35,36,39,40,45,48,49,51,55,56,57,60,63,64,
%T 65,69,72,75,77,80,81,84,85,87,88,91,93,95,96,99,100,104,105,108,111,
%U 112,115,117,119,120,121,123,125,128,129,132,133,135,136,140,141,143
%N Numbers having at least two representations as b^2 - c^2 with b > c >= 0.
%C This is the union of the squares > 4 and A306102: numbers that are the difference of two positive squares in at least two ways (where c=0 is excluded). - _M. F. Hasler_, Jul 12 2018
%F A058957 = { n | A034178(n) >= 2 }. - _M. F. Hasler_, Jul 10 2018
%e 9 is in the sequence since 9 = 3^2 - 0^2 = 5^2 - 4^2;
%e 45 is in the sequence since 45 = 7^2 - 2^2 = 9^3 - 6^2 = 23^2 - 22^2.
%t Select[Range@143, Length@ FindInstance[x^2 - y^2 == # && x>y>= 0, {x, y}, Integers, 2] == 2 &] (* _Giovanni Resta_, Jul 10 2018 *)
%o (PARI) select( is(n)=fordiv(n, d, d^2 > n && return; (n-d^2)%(2*d) || c++<2 || return(1)), [1..200]) \\ _M. F. Hasler_, Jul 10 2018
%Y Cf. A034178, A016825.
%Y Cf. A306102 (subsequence and variant using c > 0).
%K nonn
%O 1,1
%A _Henry Bottomley_, Jan 13 2001
%E Name edited by _M. F. Hasler_, Jul 10 2018
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