21060

Appears in sequences

• a(n) = floor(n(n-1)(n-2)(n-3)/20).at n=27A011930
• a(n) = (d(n)-r(n))/2, where d = A026046 and r is the periodic sequence with fundamental period (0,1,0,1).at n=46A026047
• For all n, if d is recursively applied to a(n) exactly 6 times then the fixed point of d-iteration is just reached.at n=36A036458
• Numbers k such that phi(k) and cototient(k) are squares but k is not in A054755.at n=15A054756
• Saint-Exupéry numbers: ordered products of the three sides of Pythagorean triangles.at n=15A057096
• Number of double tangents of order n.at n=15A060784
• a(n) = 27*(n-1)*(n-2)*(n-3)*(3*n-8)/2.at n=5A064197
• Integers i > 1 for which there is no prime p such that i is a solution mod p of x^4 = 2.at n=36A065903
• 1/6 the number of colorings of an n X n octagonal array with 6 colors.at n=2A068295
• Numbers k such that (k-1, k+1) and (k/2-1, k/2+1) are both pairs of twin primes.at n=10A076504
• Numbers n divisible by exactly two nontrivial permutations (rearrangements) of the digits of n.at n=23A090057
• Where A007535 reaches a record.at n=38A098653
• Indices of primes in sequence defined by A(0) = 43, A(n) = 10*A(n-1) - 27 for n > 0.at n=17A101713
• a(n) = 1*3^(3*n) + 2*3^(2*n) - 3*3^(1*n).at n=2A122038
• Triangle T(n,k), 0 <= k <= n, read by rows given by: T(0,0)=1, T(n,k)=0 if k < 0 or if k > n, T(n,0) = T(n-1,1), T(n,k) = T(n-1,k-1) + 3*T(n-1,k) + T(n-1,k+1) for k >= 1.at n=49A126970
• a(n) = sigma(sigma(n))*sigma(n).at n=39A164533
• a(n) = 65*n^2.at n=17A165798
• Numbers with prime factorization pqr^2s^4.at n=19A190107
• G.f. A(x) satisfies A(x) = 1 + 9*x*A(x)^(4/3).at n=4A214668
• Smallest k such that q=2*k*prime(n)^4+b , r=2*k*q^4+c , s=2*k*r^4+d and q, r and s are all prime numbers with b, c and d = -1 or 1.at n=37A225056