21337

Appears in sequences

• Fibonacci sequence beginning 1, 21.at n=16A022391
• Near Cullen numbers: k such that (k+1)*2^k + 1 is prime.at n=25A029544
• Semiprimes in A056109.at n=39A113528
• a(n) = A128020(n)/n.at n=18A128021
• Total number of parts of multiplicity 9 in all partitions of n.at n=45A222709
• Fundamental discriminants d uniquely characterizing all complex biquadratic fields Q(sqrt(-3),sqrt(d)) which have 3-class group of type (3,3) and second 3-class group isomorphic to SmallGroup(729,34).at n=9A250241
• Numbers n such that n = concat(a,b) and n = phi(n) + phi(a) + phi(b), with a>0 and b>0, where phi(n) is the Euler totient function of n.at n=5A258319
• Numbers of the form HMMSS with primes H < 24 and MM, SS < 60, for which the number of seconds after midnight, 3600*H+60*MM+SS, is also prime.at n=31A295011
• a(n) = 12*n^2 + 4*n + 1.at n=42A381390
• Consecutive states of the linear congruential pseudo-random number generator 20403*s mod 2^15 when started at s=1.at n=14A384196