21409

Appears in sequences

• Numbers k such that 105*2^k+1 is prime.at n=43A032402
• a(n) = a(1) + a(2) + ... + a(n-1) + a(m) for n >= 4, where m = 2*n - 2 - 2^(p+1) and p is the unique integer such that 2^p < n-1 <= 2^(p+1), with a(1) = a(2) = 1 and a(3) = 2.at n=14A049939
• Counterbalanced numbers: Composite numbers k such that phi(k)/(sigma(k)-k) is an integer.at n=23A055940
• Number of partitions of 9*n-8 into parts having in decimal representation digital root 1.at n=30A156145
• Number of UH^jU's for some j>0, where U=(1,1) and H=(1,1), in all peakless Motzkin paths of length n (can be easily expressed using RNA secondary structure terminology).at n=15A187257
• Numbers with two or more distinct prime factors such that the number and all its prime factors fall on a single straight line when they are plotted on a square spiral.at n=40A346294
• Number of integer compositions of n whose leaders of strictly decreasing runs are distinct.at n=20A374761
• a(n) = Sum_{k=0..floor(n/4)} binomial(n+k,k) * binomial(n-3*k,k).at n=15A383529