21688

Appears in sequences

• Trajectory of 1001 under "3x+1" map.at n=24A100709
• Numbers n such that 6*10^n + 4*R_n + 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n.at n=19A103036
• Expansion of g.f. -x*(5*x^7-20*x^6-2*x^5+54*x^4+7*x^3-20*x^2-8*x-1)/((x^4-x^3-3*x^2+x+1)*(x^4+x^3-3*x^2-x+1)).at n=11A122013
• E.g.f. A(x) satisfies: A(x/(1-x))/(1-x) = (1/x) * d/dx x^2*A(x)/2.at n=5A209884
• Number of length 3+3 0..n arrays with some disjoint pairs in every consecutive four terms having the same sum.at n=12A247535
• Triangle read by rows: Kreweras's "Rule A_4 left thickness" numbers.at n=38A259099
• Composites whose prime factorization in base 5 is an anagram of the number in base 5.at n=12A260049
• Number of ways to choose a multiset of strict partitions, or odd partitions, of odd numbers, whose weights sum to n.at n=27A300300
• Number of triangular regions in a "frame" of size n X n (see Comments in A331776 for definition), divided by 4.at n=21A332595
• a(n) is the first position where the digits of n occur simultaneously in the decimal expansions of Pi and e.at n=22A381980