20521

Properties

Appears in sequences

• Quadruples of different integers from [ 2,n ] with no common factors between pairs.at n=46A015628
• Numbers k such that the continued fraction for sqrt(k) has period 77.at n=24A020416
• Primes p such that p-12, p and p+12 are consecutive primes.at n=19A053072
• Numbers k such that k^2 contains only digits {1,2,4}.at n=10A053880
• a(n) = T(n,n-5), array T as in A055801.at n=37A055805
• When squared gives number composed just of the digits 1, 2, 3, 4.at n=30A061677
• Primes such that a sum of any two adjacent digits is prime; first and last digits are considered adjacent.at n=48A086244
• Maximum value taken on by f(P) = Sum_{i=1..n} p(i)*p(n+1-i) as {p(1),p(2),...,p(n)} ranges over all permutations P of {1,2,3,...,n}.at n=39A087035
• Primes in which the unit place digit is 1 and the k-th most significant digit is prime (2,3,5,7) if k is prime else is composite (4,6,8,9,0).at n=27A089704
• Prime numbers p such that p +- ((p-1)/3) are primes.at n=19A137703
• Primes of the form 10n^2+6n+1.at n=18A154409
• Noncomposite numbers in the eastern ray of the Ulam spiral as oriented on the March 1964 cover of Scientific American.at n=20A168022
• Number of permutations of length n which avoid the patterns 321 and 1324.at n=19A179257
• Prime numbers 3*n-2 such that n, 2*n-1 and 3*n-2 are prime.at n=31A180025
• Primes p such that p^2 divides 2^(2^(p-1)-1) - 1.at n=24A188465
• Fibonacci with priority for primes: a(0)=0, a(1)=1, for n >= 2, a(n) = a(n-1) + a(k), where 0 < k <= n-2 is maximal index such that a(n-1) + a(k) is prime. If there is no such k, then a(n) = a(n-1) + a(n-2).at n=30A216231
• Primes p such that p = 361 + 420*k for some k.at n=20A217656
• Prime numbers whose central digit equals the sum of the other digits.at n=16A235119
• Primes whose base-3 representation also is the base-2 representation of a prime.at n=32A235265
• Primes p such that p^4-p^3+1 and p^4-p^3-1 are also primes.at n=7A238136