21649

Properties

Appears in sequences

• Table of prime factors of 10^n - 1 (with multiplicity).at n=48A001270
• Bisection of A002470.at n=24A002286
• Smallest number with reciprocal of period length n in decimal (base 10).at n=11A003060
• Smallest primitive factor of 10^n - 1. Also smallest prime p such that 1/p has repeating decimal expansion of period n.at n=10A007138
• Divisors of 10^11 - 1.at n=3A027896
• Numbers k such that the period of the continued fraction for sqrt(k) contains exactly 88 ones.at n=27A031856
• Denominators of continued fraction convergents to sqrt(857).at n=12A042655
• Triangle of prime numbers in which n-th row lists all primes p such that 1/p has decimal period n, n >= 1.at n=14A046107
• Primes p such that p, p+12, p+24 are consecutive primes.at n=21A052188
• Primes with 14 as smallest positive primitive root.at n=15A061327
• Smallest prime factor of repunit(n) = (10^n-1)/9 (A002275).at n=9A067063
• a(1) = 1 and then primes arising in A083192. Concatenation of n-th term to (2n-1)-th term of A083192.at n=2A083193
• a(1) = 1; then the smallest number such that both the forward and reverse n-th partial concatenation is a prime for n > 1. (Reverse concatenation is taken term-wise and not digit-wise.)at n=34A083992
• a(1) = 1, then the smallest prime divisor of A065447(n) not included earlier.at n=21A087552
• Irregular triangle read by rows in which row n lists prime factors (with multiplicity) of the repunit (10^n - 1)/9 (A002275(n)).at n=27A102380
• Numbers n such that 4*10^n + 2*R_n + 5 is prime, where R_n = 11...1 is the repunit (A002275) of length n.at n=11A102986
• Primes from merging of 5 successive digits in decimal expansion of (Pi^2).at n=15A104928
• a(n) = A001333(n) - (-2)^(n-1), n > 0.at n=11A111108
• Smallest prime factor of prime(n)-th repunit number.at n=4A147555
• Primes of the form 20*k^2 + 36*k + 17.at n=14A154419