21684

Appears in sequences

• Number of positive integers <= 2^n of form 6*x^2 + 7*y^2.at n=18A054182
• Group the natural numbers such that the n-th group sum is divisible by prime(n): (1, 2, 3), (4, 5), (6, 7, 8, 9), (10, 11), (12, 13, 14, 15, 16, 17, 18, 19, 20, 21), ... Sequence contains the sum of the terms in the n-th group.at n=33A086491
• Absolute row sums of triangle A104505, which is equal to the right-hand side of the triangle A084610 of coefficients in (1+x-x^2)^n.at n=13A104508
• Matrix inverse of triangle A122177, where A122177(n,k) = C( k*(k+1)/2 + n-k + 2, n-k) for n>=k>=0.at n=40A121437
• a(n) = 2*n*(7*n + 5).at n=39A195027
• Number of 2n-bead necklaces labeled with numbers 1..7 not allowing reversal, with neighbors differing by exactly 1.at n=9A208726
• Let b(n) be the number of permutations {c_1..c_n} of {1..n} for which c_1 - c_2 + ... + (-1)^(n-1)*c_n are triangular numbers (A000217). Then a(n) = b(n)/A010551(n).at n=20A294811
• Decimal representation of binary numbers with string structure 10s00, s in {0,1}*, such that it results in a non-palindromic cycle of length 4 in the Reverse and Add! procedure in base 2.at n=19A306514
• a(n) is the decimal representation of the binary number with string structure 10s00, s in {0,1}*, such that it results in a non-palindromic cycle of length 4 in the Reverse and Add! procedure in base 2 and is not in the trajectory of any m < a(n).at n=5A306516