410296320domain: NAppears in sequencesNumber of pairs of 2 X 2 matrices over Z/nZ that commute.at n=23A227479Triangular array read by rows. T(n,k) is the number of rooted labeled trees on n nodes with exactly k vertices whose unique descendent is a leaf, n >= 1, 0 <= k <= floor((n-1)/2) + delta_{2,n}.at n=28A284417