21121

Properties

Appears in sequences

• Smallest natural number requiring n words in English (as spoken in England).at n=7A001167
• Quintan primes: p = (x^5 - y^5)/(x - y).at n=12A002649
• Expansion of g.f. 1/((1 - 7x)*(1 - 9x)).at n=4A016178
• Primes that contain digits 1 and 2 only.at n=6A020450
• Numbers whose least quadratic nonresidue (A020649) is 17.at n=15A025026
• "BIJ" (reversible, indistinct, labeled) transform of 0,1,1,1...at n=8A032110
• Primes having only {0, 1, 2} as digits.at n=21A036953
• Decimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 2,1,1.at n=4A037559
• Least prime in A031936 (lesser of 18-twins) whose distance to the next 18-twin is 2*n.at n=27A052358
• Primes with 19 as smallest positive primitive root.at n=19A061331
• Define a pair of sequences by p(0) = 0, q(0) = p(1) = q(1) = 1, q(n+1) = p(n)*q(n-1), p(n+1) = q(n+1) + q(n) for n > 0; then a(n) = p(n) and A064183(n) = q(n).at n=8A064526
• Stirling transform of (n!)^2.at n=5A064618
• Smallest prime > 2n+1 beginning and ending with 2n+1, or 0 if no such prime exists.at n=10A070278
• The zero-free, right-to-left factorial walk encoding for each rooted plane tree encoded by A014486. Sequence A071155 shown with factorial expansion (A007623).at n=38A071157
• Factorial expansion of A071156.at n=37A071158
• Smallest prime beginning and ending in 2n+1 or 0 if no such prime exists.at n=10A071234
• a(1) = 2, a(n+1) is the largest squarefree number < 2*a(n).at n=15A076994
• 8th binomial transform of (0,1,0,1,0,1,....), A000035.at n=5A081202
• Left-to-right binary enumeration.at n=39A081242
• Primes in which the digit string can be partitioned into three parts such that third (least significant) part is the product of the first two.at n=16A088294