20995

Appears in sequences

• Numbers n such that phi(n + 1) | sigma(n) for n congruent to 1 (mod 3).at n=39A015817
• Numbers k such that the period of the continued fraction for sqrt(k) contains exactly 15 ones.at n=26A031783
• Numbers n such that 115*2^n-1 is prime.at n=21A050583
• a(n) = Sum_{1 <= x, y <= n} lcm(x, y).at n=17A064951
• Triangle read by rows: T(k,s) = binomial(k+s,2s+1)*(2k-1)*(2k+1)/(2s+3), k >= 1, 0 <= s <= k-1.at n=40A111126
• Triangle read by rows: T(k,s)=(2k-1)(2k+1)binomial(2k-s-1,2k-2s-1)/(2k-2s+1); k>=1, 0<=s<=k-1.at n=40A111127
• a(n) = 14 + floor( (1 + Sum_{j=0..n-1} a(j)) / 2).at n=18A120141
• a(n) = product of those positive integers which are coprime to both n and n+1 and which are <= n.at n=20A124740
• Number of ways to place zero or more nonadjacent 2,1 2,2 3,0 3,1 3,3 4,2 4,3 polyhexes in any orientation on a planar nXnXn triangular grid.at n=7A155353
• Numbers k such that 3k-4, 3k-2, 3k+2, and 3k+4 are primes.at n=27A173092
• The product of primes <= n that are strongly prime to n.at n=22A181836
• Number of 4-step one space for components leftwards or up, two space for components rightwards or down asymmetric quasi-bishop's tours (antidiagonal moves become knight moves) on an n X n board summed over all starting positions.at n=22A187608
• The Wiener index of the straight pentachain of n pentagonal rings (see Fig. 2.1 in the A. A. Ali et al. reference).at n=20A224459
• Product of primes appearing in the factorization of n! with even exponents.at n=38A240502
• Expansion of sqrt( (1+x + sqrt(1-18*x+x^2)) / (2*(1-18*x+x^2)) ).at n=4A243946
• Number of (n+2)X(1+2) 0..3 arrays with every 3X3 subblock row and column sum prime and every diagonal and antidiagonal sum nonprime.at n=7A251943
• T(n,k)=Number of (n+2)X(k+2) 0..3 arrays with every 3X3 subblock row and column sum prime and every diagonal and antidiagonal sum nonprime.at n=28A251950
• T(n,k)=Number of (n+2)X(k+2) 0..3 arrays with every 3X3 subblock row and column sum prime and every diagonal and antidiagonal sum nonprime.at n=35A251950
• Numbers whose arithmetic derivative is equal to their BCR, where BCR = A036044, binary-complement-and-reverse = take one's complement then reverse bit order.at n=6A269633
• Triangle read by rows, T(n,k) = denominator(binomial(1/2,n-k))*binomial(n+1/2, k+1/2), for n>=0 and 0<=k<=n.at n=50A269950