20789

Properties

Appears in sequences

• Numbers k such that the continued fraction for sqrt(k) has odd period and if the last term of the periodic part is deleted the two central terms are both 11.at n=23A031599
• a(1) = 1, then add, multiply, subtract, multiply 2, 3, 4, 5; 6, 7, 8, 9; ... in that order.at n=11A077384
• Table read by rows where i-th row consists of primes P of the form P=j*P(i)# -1 or P=j*P(i)# +1 with 0 < j < P(i+1). Here P(r)# = A002110.at n=39A087715
• Indices of primes in sequence defined by A(0) = 37, A(n) = 10*A(n-1) - 13 for n > 0.at n=11A101837
• Highly cototient numbers that are prime, or intersection of A000040 and A100827.at n=36A105440
• Primes congruent to 49 mod 61.at n=33A142847
• Primes p of the form 4*k+1 for which s=26 is the least positive integer such that s*p-(floor(sqrt(s*p)))^2 is a square.at n=26A145050
• Least number k>1 such that (tau(k-1)+tau(k+1))/tau(k) = n where tau = A000005.at n=34A190644
• G.f.: A(x) = x + Sum_{n>=1} x^(2*n) / (1+x)^A193259(n).at n=15A193263
• Sophie Germain primes that are also highly cototient numbers.at n=16A209194
• Primes of the form 2*k!! - 1.at n=4A215780
• Primes q = 4*p+1, where p == 2 (mod 5) is also prime.at n=35A221981
• E.g.f.: exp( Sum_{n>=1} x^(2*n-1) / (n*(2*n-1)) ).at n=9A222055
• Primes of form n^2 + 625.at n=29A256777
• Primes of the form A060735(k) +- 1, where A060735 lists multiples of primorials (A002110) less than the next larger primorial.at n=38A257658
• Primes 8k + 5 preceding the maximal gaps in A269513.at n=13A269514
• Primes of the form 11*n^2 + 55*n + 43.at n=32A292578
• 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=26A293711
• 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=28A293713
• Prime time primes on 6-digit clocks, second definition: primes of the form HMMSS where H, MM, SS are primes, H < 24, MM and SS < 60.at n=22A295013