21431

Appears in sequences

• Analog of A006684 for the 7x+1 problem (cf. A133421).at n=19A133425
• Number of partitions p of n such that the number of distinct parts is not a part and max(p) - min(p) is a part.at n=51A241388
• Number of n X 2 0..1 arrays with each 1 adjacent to 1 or 3 king-move neighboring 1s.at n=10A295937
• a(1) = 1; a(n) = Sum_{k=1..n-1} a(n-k) * Sum_{d|k} a(d)*a(k/d).at n=8A307816
• a(n) = Sum_{k=0..floor(n/3)} Stirling2(n - 2*k,n - 3*k).at n=17A357925
• Odd semiprimes k = p*q such that k = A325820(p,q), with p, q primes > 3, and A325820 is the carryless base-3 multiplication.at n=49A391331