19521

Appears in sequences

• Hexanacci numbers with a(0) = ... = a(5) = 1.at n=18A000383
• Hyperperfect numbers: k = m*(sigma(k) - k - 1) + 1 for some m > 1.at n=11A007592
• 2-hyperperfect numbers: n = 2*(sigma(n) - n - 1) + 1.at n=2A007593
• Odd octagonal numbers: (2n+1)*(6n+1).at n=40A014641
• Quotients k*(k+1)*(k+2) / (k+(k+1)+(k+2)) that are lucky numbers.at n=16A032792
• Hyperperfect numbers: x such that x = 1 + k*(sigma(x)-x-1) for some k > 0.at n=15A034897
• Numerators of continued fraction convergents to sqrt(721).at n=11A042388
• a(1) = a(2) = 1, a(3) = 9; for n > 3, a(n) = 3*a(n-1) - 3*a(n-2) + 9*a(n-3).at n=9A101990
• Sum 3^max(k,n-k),k=0..n.at n=8A107660
• n+phi(n)+phi(phi(n)) is a cube.at n=18A116042
• Nonsemiprime hyperperfect numbers.at n=4A133447
• Binomial transform of A131666.at n=11A135254
• Octagonal numbers which are the sums of exactly two positive octagonal numbers.at n=14A136346
• Number of n X n binary arrays symmetric about the diagonal and under 90 degree rotation with all ones connected only in a 01000-11111-01000 pattern in any orientation.at n=24A147024
• 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, 0), (-1, -1, 1), (-1, 1, 1), (1, 0, -1), (1, 1, 1)}.at n=8A149618
• Quasi-antiperfect numbers.at n=5A192287
• Number of singular 2 X 2 matrices having all elements in {-n,...,n}.at n=21A209981
• Consider the aliquot parts, in ascending order, of a composite number. Take their sum and repeat the process deleting the minimum number and adding the previous sum. The sequence lists the numbers that after some iterations reach a sum equal to themselves.at n=7A246544
• Number of (n+1)X(4+1) arrays of permutations of 0..n*5+4 with each element having directed index change -2,0 -1,0 0,-1 or 1,1.at n=6A264624
• T(n,k)=Number of (n+1)X(k+1) arrays of permutations of 0..(n+1)*(k+1)-1 with each element having directed index change -2,0 -1,0 0,-1 or 1,1.at n=51A264628