21667

Appears in sequences

• Pseudoprimes to base 21.at n=37A020149
• Strong pseudoprimes to base 14.at n=9A020240
• Strong pseudoprimes to base 20.at n=11A020246
• Strong pseudoprimes to base 22.at n=13A020248
• Strong pseudoprimes to base 41.at n=16A020267
• Strong pseudoprimes to base 45.at n=8A020271
• Strong pseudoprimes to base 68.at n=25A020294
• a(n) = (2*n-1)*(5*n^2-5*n+6)/6.at n=23A063489
• a(2n) = concatenation of 4n+1 and 4n+2, a(2n+1) = concatenation of the two most nearly equal numbers whose product is a(2n).at n=17A068517
• Nonprimes k such that k divides 3^(k-1) - 2^(k-1).at n=38A073631
• Numbers k such that 10^999 + k is a (titanic) prime.at n=13A074282
• a(n) = least semiprime with factors not previously used containing integers 2n and 2n+1 as substrings.at n=23A086887
• Number of walks within N^3 (the first octant of Z^3) starting at (0,0,0) and consisting of n steps taken from {(-1, -1, 1), (-1, 0, 0), (0, 0, 1), (0, 1, -1), (1, 0, -1)}.at n=10A148564
• Index sequence for limit-block extending A000002 (Kolakoski sequence) with first term as initial block.at n=41A246145
• Numbers k such that 3^(k-1) == 2^(k-1) !== 1 (mod k).at n=27A285300
• Prime generating polynomial: a(n) = (4*n - 29)^2 + 58.at n=43A320772
• G.f.: Sum_{k>=0} x^k * Product_{j=1..3*k} (1 + x^j)/(1 - x^j).at n=22A385089