20180

Appears in sequences

• a(n) is the concatenation of n and 9n.at n=19A009474
• Fibonacci iteration starting with (1, a(n)) leads to a "nine digits anagram".at n=32A034587
• Triangle of numbers arising in enumeration of walks on cubic lattice.at n=40A052179
• Interprimes which are of the form s*prime, s=20.at n=21A075295
• Expansion of 2*x^2*(7+14*x+4*x^2)/((1+x-x^2)*(1-2*x-10*x^2-6*x^3)).at n=6A120714
• a(1)=1, a(n) = a(n-1) + n^5 if n odd, a(n) = a(n-1) + n^0 if n is even.at n=7A140162
• a(n) = 961*n - 1.at n=20A158412
• G.f. satisfies: A(x) = exp( Sum_{n>=1} (Sum_{k=0..n} C(n,k)^3*A(x)^k) * x^n/n ).at n=6A192131
• Number of positive walks with n steps {-4,-3,-2,-1,1,2,3,4} starting at the origin, ending at altitude 2, and staying strictly above the x-axis.at n=7A277923
• Numbers n such that there are precisely 15 groups of orders n and n + 1.at n=8A295995
• Expansion of 1/(theta_3(q) * theta_3(q^2) * theta_3(q^3)), where theta_3() is the Jacobi theta function.at n=22A320070