21781

Appears in sequences

• Pseudoprimes to base 17.at n=41A020145
• Strong pseudoprimes to base 17.at n=14A020243
• a(n) = (27*n^2 + 9*n + 2)/2.at n=40A093485
• a(n) = smallest squarefree number not less than a(n-1)+a(n-2), a(1)=1, a(0)=0.at n=21A118728
• a(n) = 20*n^2 + 1.at n=33A158493
• Triangle T(n,k) read by rows: T(n,1) = T(n,n)=1, otherwise T(n,k) = (3n-3k+1)*T(n-1,k-1) + k*(3k-2)*T(n-1,k), 1<=k<=n.at n=34A166962
• Semiprime centered triangular numbers.at n=44A184481
• Semiprimes of the form 5*n^2 + 1.at n=22A212707
• Numbers n such that Q(sqrt(n)) has class number 9.at n=33A218041
• Fundamental discriminants of real quadratic number fields with class number 9.at n=22A218159
• Number of nX(n+1) arrays of permutations of n+1 copies of 0..n-1 with every element equal to or 1 greater than any north or northeast neighbors modulo n and the upper left element equal to 0.at n=6A267554
• Number of 7Xn arrays containing n copies of 0..7-1 with every element equal to or 1 greater than any north or northeast neighbors modulo 7 and the upper left element equal to 0.at n=7A267561
• Convolution of A000217 and A001045.at n=14A272144
• a(n) = (3 * a(n-3) + a(n-1) * a(n-5)) / a(n-6), a(0) = a(1) = ... = a(5) = 1.at n=17A275176
• Decimal representation of the x-axis, from the origin to the right edge, of the n-th stage of growth of the two-dimensional cellular automaton defined by "Rule 155", based on the 5-celled von Neumann neighborhood.at n=14A279253
• Decimal representation of the x-axis, from the left edge to the origin, of the n-th stage of growth of the two-dimensional cellular automaton defined by "Rule 179", based on the 5-celled von Neumann neighborhood.at n=14A279666
• Decimal representation of the x-axis, from the left edge to the origin, of the n-th stage of growth of the two-dimensional cellular automaton defined by "Rule 195", based on the 5-celled von Neumann neighborhood.at n=14A279750
• Decimal representation of the x-axis, from the left edge to the origin, of the n-th stage of growth of the two-dimensional cellular automaton defined by "Rule 219", based on the 5-celled von Neumann neighborhood.at n=14A279957
• Decimal representation of the x-axis, from the origin to the right edge, of the n-th stage of growth of the two-dimensional cellular automaton defined by "Rule 324", based on the 5-celled von Neumann neighborhood.at n=14A281105
• a(n) = -1/3 * (u^n-1)*(v^n-1) with u = 1+sqrt(3), v = 1-sqrt(3).at n=10A305650