21401

Properties

Appears in sequences

• Smallest number with reciprocal of period length n in decimal (base 10).at n=25A003060
• Smallest primitive factor of 10^n - 1. Also smallest prime p such that 1/p has repeating decimal expansion of period n.at n=24A007138
• Primes p from A031924 such that A052180(primepi(p)) = 17.at n=25A052234
• Second term of weak prime sextet: p(m)-p(m-1) < p(m+1)-p(m) < p(m+2)-p(m+1) < p(m+3)-p(m+2) < p(m+4)-p(m+3).at n=6A054829
• Primes p such that x^5 = 2 has a solution mod p, but x^(5^2) = 2 has no solution mod p.at n=15A070182
• Primes p such that the period of the decimal expansion of 1/p is a square.at n=28A072858
• Numbers k such that k, sigma(k) and phi(k) have the same decimal digits (ignoring multiplicity).at n=32A082059
• Primes p for which the period length of 1/p is a perfect power, A001597.at n=36A128948
• Primes dividing numbers k such that k divides the k-th Lucas number A000032(k).at n=5A129729
• Primes p such that the multiplicative order of 2 modulo p is (p-1)/10.at n=32A152310
• Primes p such that 2*p^4-+21 are also prime.at n=30A174367
• Primes dividing repunits R(10^n) for some n.at n=27A178070
• Primes p such that the decimal representation of 1/p does not contain every digit 0-9.at n=26A187614
• Prime factors of 10^100 - 1.at n=10A200861
• Numbers n such that 10^n - 1 divides 10^(10^100) - 10.at n=37A200879
• Number of partitions of n+7 with largest inscribed rectangle having area <= n.at n=30A218628
• Prime numbers whose central digit equals the sum of the other digits.at n=17A235119
• Intersection of A251964, A252280 and A252281.at n=34A252283
• Primes p such that 2*p + 47 is a square.at n=43A269788
• Artiads (A001583) congruent to 1 mod 50 and having 2 as a quintic residue.at n=4A270799