210209

Properties

Appears in sequences

• Primes whose decimal expansion is a concatenation of two or more consecutive decreasing numbers (no leading zeros allowed).at n=27A052088
• Primes formed by concatenating k with k-1.at n=25A052089
• Primes p such that 32p+1 and (p-1)/32 are both prime.at n=20A086476
• Primes p such that 2p+1, 4p+3, 6p+5, 8p+7 are all primes.at n=12A107021
• Primes p such that 2p+1, 4p+3, 6p+5, 8p+7, 10p+9 are all primes.at n=2A107022
• Primes that are the concatenation of k and k-1 for some k, where the concatenation of k-2 and k-3 is also prime.at n=3A156120
• Sophie Germain primes that are also highly cototient numbers.at n=24A209194
• a(n) = (A242804(n)-9)/12.at n=21A257044
• Numbers k such that phi(sigma(k))/k < phi(sigma(m))/m for all m < k, where sigma is the sum of divisors function (A000203) and phi is Euler's totient function (A000010).at n=32A293711
• Numbers k such that phi(psi(k))/k < phi(psi(m))/m for all m < k, where phi is Euler's totient function (A000010) and psi is the Dedekind psi function (A001615).at n=34A293713
• Numbers k where records occur for phi(k)/phi(k+1), where phi is the Euler totient function (A000010).at n=28A335070
• a(n) is the least number whose sum of digits in primorial base equals n.at n=41A343048
• Prime numbersat n=18828