21489

Appears in sequences

• a(n) = (2*n-1)*(3*n-1)*(4*n-1).at n=10A033589
• One ninth of deca-factorial numbers.at n=3A035278
• a(1)=1, a(2)=2, a(3)=3; for n >= 3, a(n) is smallest number such that all a(i) for 1 <= i <= n are distinct, all a(i)+a(j) for 1 <= i < j <= n are distinct and all a(i)+a(j)+a(k) for 1 <= i < j < k <= n are distinct.at n=25A036241
• a(0) = 0; a(1)=1; for n>1, a(n) = least positive integer m not among a(1),...,a(n-1) such that |m-a(n-1)| > |a(n-1)-a(n-2)|.at n=39A078783
• A sequence derived from pentagonal numbers, or a Stirling number of the first kind matrix.at n=18A094952
• Number at end of segment n of A078783.at n=12A117072
• a(n) = (n^3 + 3*n - 2)/2.at n=34A132127
• Numbers k such that k and k+1 have 4 distinct prime factors.at n=32A140078
• Numbers n such that sigma(n)/phi(n) = 25/9, where sigma = A000203, phi = A000010.at n=2A165630
• Triangle read by rows: T(n,k) is the number of partitions of the set {1,2,...,n} having exactly k blocks that do not consist of consecutive integers (0<=k<=floor(n/2); a singleton is considered a block of consecutive integers).at n=37A177256
• 13 times hexagonal numbers: a(n) = 13*n*(2*n-1).at n=29A194713
• Numbers such that the sum of the largest and the smallest prime divisor equals the sum of the other distinct prime divisors.at n=31A199745
• Primitive (squarefree) elements of A199745.at n=16A200145
• Squarefree numbers which yield zero when their prime factors are xored together.at n=13A235488
• Product of all primes p such that 2n - p is also prime.at n=14A238711
• Number of length 3+2 0..n arrays with the sum of second differences squared multiplied by some arrangement of +-1 equal to zero.at n=17A250322
• a(n) = n*(n+1)*(22*n-19)/6.at n=18A256716
• a(n) = n*(n+1)*(n+2)*(n^2+2*n+17)/120.at n=17A257199
• Indices of the start of 10 successive distinct digits in the decimal expansion of e (2.718281828...).at n=18A258166
• a(n) = n*(67*n - 89)/2.at n=26A263227