20441

Properties

Appears in sequences

• Number of equivalence classes of base-3 necklaces of length n, where necklaces are considered equivalent under both rotations and permutations of the symbols.at n=13A002076
• Primes of the form k^2 - 8.at n=32A028886
• Numbers k such that the continued fraction for sqrt(k) has odd period and if the last term of the periodic part is deleted then there are a pair of central terms both equal to 9.at n=32A031422
• Initial terms of '4-block' primes as described in A032591.at n=30A032592
• Luhn primes: primes p such that p + (p reversed) is also a prime.at n=39A061783
• a(n) = floor(e*(n+3)!) - (n+3)*(n+2)*(n+1)*n*floor(e*(n-1)!).at n=24A080770
• a(n) = p is the smallest prime introducing a consecutive prime-difference pattern as follows: [2,2n,2], i.e., [p, p+2, p+2+2n, p+2+2n+2] are consecutive primes. Increasing middle prime gap in the immediate neighborhood of two small gaps(=2); a(n) = 0 if no such pattern exists.at n=16A082512
• Twin-prime-indexed primes (TWIPS): members of a pair of twin primes whose prime index is also a member of a pair of twin primes.at n=39A087373
• Lower twin primes with lower twin prime index.at n=20A088460
• Primes of the form x^3+x^2+x+2.at n=9A088547
• Initial members of 25 consecutive primes in a 5 X 5 spiral wherein the mean of all 12 sums is prime.at n=35A094458
• Row sums of array A097306.at n=41A097307
• Numbers n such that (n + prime(n)), (n+1 + prime(n+1)), (n+2 + prime(n+2)) and (n+3 + prime(n+3)) are divisible by 5.at n=13A107582
• Numbers k such that phi(k) = sigma(phi(phi(k))).at n=7A115448
• Primes congruent to 6 mod 61.at n=36A142804
• Triangle T(n,k) read by rows: Sum_{k=0..binomial(n,2)} T(n,k)*q^k = n!*Sum_{pi} faq(n,q)/Product_{i=1..n} e(i)!*faq(i,q)^e(i), where pi runs over all nonnegative integer solutions to e(1) + 2*e(2) + ... + n*e(n) = n and faq(i,q) = Product_{j=1..i} (q^j-1)/(q-1), i = 1..n.at n=31A152474
• Chen primes A109611(k) which have the same sum-of-digits as their index k.at n=38A176012
• a(n) = n^3 - 2*n^2 + 2*n + 1.at n=27A188947
• Least n-gap prime: a(n) = least prime p for which there is no prime between n*p and n*q, where q is the next prime after p.at n=30A195325
• Number of (w,x,y,z) with all terms in {1,...,n} and w<2x+y+z.at n=12A212145