544544domain: NAppears in sequencesNumber of pairs of orthogonal (-x,y) vectors of length 1*(x+y), where x/y is the n-th rational <= 1, ordered first by y and then x, e.g. 1/1, 1/2, 1/3, 2/3, 1/4, 3/4 ...at n=25A225986a(n) = denominator(Sum_{k=1..n} (2^(k-1) - 1)/k).at n=34A330719