21490

Appears in sequences

• Number of 4-line partitions of n decreasing across rows.at n=25A003292
• Weird numbers: abundant (A005101) but not pseudoperfect (A005835).at n=37A006037
• Unitary weird numbers: unitary abundant (A034683) but not unitary pseudoperfect (A293188).at n=35A064114
• 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=41A078783
• Number of partitions of 2*n into distinct parts with exactly two odd parts.at n=37A096914
• Number at middle of segment n of A078783.at n=13A117071
• a(n) = prime(prime(prime(prime(n) - 1) - 1) - 1) - 1, where prime(n) is the n-th prime.at n=20A141217
• 1/4 the number of permutations of 3 indistinguishable copies of 1..n with exactly 3 local maxima.at n=3A152500
• a(n) = n*(16*n^2 + 3*n - 13)/6.at n=20A172078
• a(n) = 3*a(n-1) - 2*a(n-2) with a(0)=28 and a(1)=70.at n=9A182465
• a(n) = n*(n^2 + 3)/2.at n=35A229183
• Numbers k such that k divides 1 + Sum_{j=1..k} prime(j)^9.at n=17A232964
• E.g.f.: exp( Sum_{n>=1} H(n) * x^(2*n)/(2*n) ) where H(n) is the n-th harmonic number.at n=4A235685
• Numbers n such that n^2 + 1 has two distinct prime divisors less than n.at n=24A263876
• Number T(n,k) of ordered set partitions of [n] where the maximal block size equals k; triangle T(n,k), n>=0, 0<=k<=n, read by rows.at n=40A276922
• Number of ordered set partitions of [2n] where the maximal block size equals n.at n=4A276923
• Numbers that are the sum of 10 consecutive primes and also the sum of 10 consecutive semiprimes.at n=5A284102
• Number of ordered set partitions of [n] where the maximal block size equals four.at n=4A320760
• Nonexponential weird numbers: nonexponential abundant numbers (A348604) that are not equal to the sum of any subset of their nonexponential divisors.at n=31A348631
• (1+e)-weird numbers: (1+e)-abundant numbers k such that no subset of the aliquot (1+e)-divisors of k sums to k.at n=32A349285