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(C99)
#include <stdio.h>
#include <math.h>
typedef unsigned long long U64;
int isTriang(U64 x) {
x+=x;
U64 r = sqrt(x);
return (r*(r+1)==x);
}
int main() {
for (U64 n=0, i=0; i < (1ULL<<32); ++i) {
U64 s, d, d1, d2, d3;
s = sqrt(n+=i);
d1 = n - s*s;
if (!isTriang(d1)) continue;
d2 = (s+1)*(s+1) - n;
if (!isTriang(d2)) continue;
d3 = (s+2)*(s+2) - n;
if (s) {
d = n - (s-1)*(s-1);
if (d < d3) d3 = d;
}
if (isTriang(d3)) printf("%llu, ", n);
}
}
(Haskell)
a232608 n = a232608_list !! (n-1)
a232608_list = filter f $ tail a000217_list where
f x = all ((== 1) . a010054) $ init $ sort $
map (abs . (x -) . (^ 2) . (+ (a000196 x))) [-1..2]
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