21945

Appears in sequences

• Odd primitive abundant numbers.at n=29A006038
• Expansion of g.f. x*(1 + x)*(1 + 6*x + x^2)/(1 - x)^7.at n=9A006858
• E.g.f.: sech(arcsin(x)+log(x+1))=1-4/2!*x^2+6/3!*x^3+53/4!*x^4-310/5!*x^5...at n=7A012910
• [ (4th elementary symmetric function of S(n))/(2nd elementary symmetric function of S(n)) ], where S(n) = {first n+3 positive integers congruent to 1 mod 4}.at n=14A024389
• 2nd elementary symmetric function of the first n+1 positive integers congruent to 2 mod 3.at n=10A024391
• Denominator of |Bernoulli(2n+2)| - |Bernoulli(2n)|.at n=9A029765
• a(n) = (2*n-1)*(3*n-1)*(4*n-1)*(5*n-1).at n=4A033590
• One third of quartic factorial numbers.at n=4A034176
• Number of partitions satisfying cn(0,5) + cn(2,5) <= cn(1,5) and cn(0,5) + cn(2,5) <= cn(4,5) and cn(0,5) + cn(3,5) <= cn(1,5) and cn(0,5) + cn(3,5) <= cn(4,5).at n=49A039883
• Odd numbers with exactly 5 distinct prime factors.at n=2A046391
• Tritriangular numbers: a(n) = binomial(binomial(n,2),2) = n*(n+1)*(n-1)*(n-2)/8.at n=21A050534
• Numbers k such that 75*2^k-1 is prime.at n=38A050563
• Denominators of column 3 of table described in A051714/A051715.at n=18A051721
• Denominators of column 3 of table described in A051714/A051715.at n=17A051721
• Consider 2n tennis players; a(n) is the number of matches needed to let every possible pair play each other.at n=9A062346
• a(n) = n^4 - (n-1)^4 + (n-2)^4 - ... 0^4.at n=14A062392
• Triangular numbers with sum of digits = 21.at n=16A068131
• Triangular numbers of the form 21*k.at n=39A069499
• Terms of A073872 that do not change their position in the rearrangement; i.e., values of A073872(n) which equal n(n+1)/2.at n=15A073873
• Odd primitive numbers such that n! divided by product of factorials of all proper divisors of n is not an integer.at n=26A075460