21777

Appears in sequences

• sech(sinh(x)+log(x+1))=1-4/2!*x^2+6/3!*x^3+53/4!*x^4-310/5!*x^5...at n=7A013024
• If p(x) is the x-th prime, then the n-th set of 4 consecutive sexy prime pairs starts at p(a(n)).at n=29A095963
• a(n) = C(n,6) + C(n,5) + C(n,4) + C(n,3) + C(n,2) + C(n,1).at n=17A115567
• Index of first occurrence of n in A165633.at n=24A165765
• Number of (w,x,y,z) with all terms in {1,...,n} and 3w = x + y + z + n + 1.at n=43A212251
• Number of length 2+2 0..n arrays with the sum of the maximum minus the minimum of adjacent triples multiplied by some arrangement of +-1 equal to zero.at n=15A252178
• Number of nX4 0..1 arrays with every element equal to 1, 2 or 3 horizontally, diagonally or antidiagonally adjacent elements, with upper left element zero.at n=5A302220
• T(n,k)=Number of nXk 0..1 arrays with every element equal to 1, 2 or 3 horizontally, diagonally or antidiagonally adjacent elements, with upper left element zero.at n=41A302224
• Number of 6Xn 0..1 arrays with every element equal to 1, 2 or 3 horizontally, diagonally or antidiagonally adjacent elements, with upper left element zero.at n=3A302229
• Increasingly larger (in absolute value) extrema of the Mertens function A002321 between subsequent zeros.at n=44A304242
• Number of tripling steps to reach 1 in the 3x+1 (Collatz) problem starting with the n-th Mersenne prime.at n=19A390817
• a(n) = Sum_{k=0..floor(n/2)} binomial(3*k,2*n-4*k).at n=15A392433