21094

Appears in sequences

• Number of primitive (period n) periodic palindromic structures using exactly four different symbols.at n=17A056521
• Sum of first n 7-almost primes.at n=24A086059
• Expansion of (1 - x)*(1 + 2*x) / ((1 + x)*(1 - 4*x - x^2)).at n=7A102129
• Numbers n such that sigma(n) and sigma(sigma(n)) are both perfect squares.at n=25A134263
• Number of planar triangular n X n X n nonnegative integer grids symmetric under 120 degree rotation with every similarly oriented 5 X 5 X 5 subtriangle summing to 14.at n=7A154090
• Triangle, read by rows, given by [0,1,1,1,1,1,1,1,...] DELTA [1,0,1,0,2,0,3,0,4,0,5,0,6,0,...] where DELTA is the operator defined in A084938.at n=49A173050
• p-INVERT of A175676 (starting at n=3), where p(S) = 1 - S - S^2.at n=14A289844
• Upper midsequence of the Fibonacci numbers (1,2,3,5,8,...) and Lucas numbers (1,3,4,7,11,...); see Comments.at n=20A355325