422076domain: NAppears in sequences(x,y) = (a(n),a(n+1)) are the solutions of (t(x)+t(y))/(1+xy) = t(2) = 3, where t(n) denotes the n-th triangular number t(n) = n(n+1)/2.at n=7A065928Number of ways the set {1,2,...,n} can be split into two subsets of which the sum of one is twice the sum of the other.at n=26A113035