OFFSET
1,1
COMMENTS
Side lengths where both triangular numbers are the same (A053141) are not included. - R. J. Mathar, Feb 11 2018
REFERENCES
Albert H. Beiler, Recreations in the theory of numbers, New York, Dover, (2nd ed.) 1966, p. 197, nr. 8.
LINKS
R. J. Mathar, Table of n, a(n) for n = 1..58
EXAMPLE
a(2) = 18, since the pair of triangular numbers 171 = 18*(18+1)/2 and 105 = 14*(14+1)/2 produce the sum 276 = 23*(23+1)/2 and the difference 66 = 11*(11+1)/2 which are both triangular numbers.
PROG
(PARI) lista(nn) = {v = vector(nn, n, n*(n+1)/2); for (n=2, nn, for (k=1, n-1, if (ispolygonal(v[n]+v[k], 3) && ispolygonal(v[n]-v[k], 3), print1(n, ", ")); ); ); } \\ Michel Marcus, Jan 08 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Martin Renner, Jan 20 2012
STATUS
approved