31021

Appears in sequences

• Squares written in base 4.at n=29A001739
• Pseudoprimes to base 15.at n=36A020143
• Pseudoprimes to base 90.at n=38A020218
• Strong pseudoprimes to base 15.at n=7A020241
• Strong pseudoprimes to base 21.at n=14A020247
• Strong pseudoprimes to base 36.at n=25A020262
• Strong pseudoprimes to base 38.at n=23A020264
• Strong pseudoprimes to base 39.at n=17A020265
• Strong pseudoprimes to base 55.at n=16A020281
• Strong pseudoprimes to base 68.at n=29A020294
• Strong pseudoprimes to base 77.at n=11A020303
• a(n) = a(1)*a(n-1) + a(2)*a(n-2) + ... + a(n-3)*a(3) for n >= 4, with initial terms 0, 1, 1, 0.at n=24A025250
• Variant of Lucas numbers: a(n) = a(n-1) + 4*a(n-2) starting with a(0)=2 and a(1)=1.at n=11A072265
• A partial product representation of A006131 and A072265.at n=10A072270
• Duplicate of A072265.at n=11A085488
• Nonprime numbers k such that k divides 3^((k+1)/2) - 2^((k+1)/2) - 1.at n=10A130062
• Numerators of partial sums of a certain alternating series of inverse central binomial coefficients.at n=6A145557
• Counting integers normally (1, 2, 3, 4, 5...), write them as roman numerals (I, II, III, IV, V...), describe them (one 1, two 1s, three 1s, one 1 one 5, one 5...), and write them out as numbers (11, 21, 31, 1115, 15...).at n=31A180105
• The least number s having exactly n fours in the continued fraction of sqrt(s).at n=23A206584
• Euler pseudoprimes to base 6: composite integers such that abs(6^((n - 1)/2)) == 1 mod n.at n=29A262053