20989

Appears in sequences

• Numbers k such that the period of the continued fraction for sqrt(k) contains exactly 90 ones.at n=16A031858
• a(n) is the smallest composite number c such that A002110(n) + c is prime.at n=33A038771
• Composite numbers which in base 6 contain their largest proper factor as a substring.at n=9A063156
• Duplicate of A063156.at n=9A063876
• a(n) = prime(n)*prime(n+2).at n=33A090076
• Triangle of numbers related to the generalized Catalan sequence C(3;n+1) = A064063(n+1), n>=0.at n=24A115154
• Fourth diagonal (M=4) of triangle A115154 (called Y(3,1)).at n=3A115190
• Ulam's spiral (SSE spoke).at n=36A143839
• Number of n X n binary arrays with all ones connected only in a 11100-00100-00111 pattern in any orientation.at n=8A147247
• Number of n X n binary arrays symmetric under horizontal and vertical reflection with all ones connected only in a 11100-00100-00111 pattern in any orientation.at n=18A147249
• Number of n X n binary arrays symmetric under horizontal and vertical reflection with all ones connected only in a 11100-00100-00111 pattern in any orientation.at n=19A147249
• a(n) = prime(n) times the n-th nonnegative noncomposite.at n=35A176098
• S_5 sequence in partition of integers > 1 described in A240521.at n=40A240522
• Semiprimes whose prime factors are of equal binary length and which differ from each other in exactly three bit positions.at n=37A261075
• Sequence of pairwise relatively prime numbers of class P_4 (see comment in A275246).at n=17A275248
• Numbers k such that k and k+2 are both A000120-perfect numbers (A175522).at n=25A360639