21584

Appears in sequences

• Numbers k such that sigma(k) = sigma(k+10).at n=26A015880
• Numbers k such that (k+3, k+5, k+17, k+257, k+65537) are all primes.at n=19A063799
• Numbers n such that phi(n) = sigma(n) - sigma(n+1).at n=4A063943
• Numbers k such that phi(k) + phi(k+1) = k.at n=16A067796
• Numbers k such that phi(k) = Sum_{d|k} core(d) where core(x) is the squarefree part of x (A007913).at n=14A074786
• XOR difference triangle, read by rows, of A099898 (in leftmost column) such that the main diagonal equals A099898 shift left and divided by 4.at n=34A099897
• Number of ballot sequences of length n with exactly 5 fixed points.at n=14A239116
• The curvature (rounded down) of touching circles inscribed in a special way in the smaller segment of circle of radius 10/9 divided by a chord of length 4/3.at n=14A247512
• Numbers which are representable as a sum of nineteen but no fewer consecutive nonnegative integers.at n=23A270303
• Numbers k such that 8k+1, 12k+1 and 24k+1 are primes and the last two are also of the form x^2 + 27y^2, so the tetrahedral number T(24k+1) is a Fermat pseudoprime to base 2.at n=13A321867
• Number of compositions (ordered partitions) of n into hexagonal pyramidal numbers (A002412).at n=47A322854
• a(n) = 4*(23 - 17*n + 8*n^2).at n=26A387458
• Numbers k such that sigma(k) = psi(k) + phi(k).at n=13A389478