20999

Appears in sequences

• Lucas-Carmichael numbers: squarefree composite numbers k such that p | k => p+1 | k+1.at n=11A006972
• Composite numbers k that divide Fibonacci(k+1).at n=10A069107
• Numbers k such that gcd(3k,8^k+1) = 3 but k does not divide the numerator of B(2k) (the Bernoulli numbers).at n=32A070193
• Numbers k that divide Fibonacci(k+1) but do not divide Fibonacci(k) + 1.at n=8A094412
• a(n) = n*(n+1)*(n^2 + 21*n + 50)/24.at n=21A101854
• a(n) = prime(n)*prime(n+1) + prime(n) + prime(n+1).at n=33A126199
• Composite numbers k that divide both Fibonacci(k+1) and Fibonacci(2k+1)-1.at n=9A182504
• Composite numbers k that divide Fibonacci(k+1) or Fibonacci(k-1).at n=22A182554
• L.g.f.: (-1/3)*log( Sum_{n>=0} (2*n+1)*(-x)^(n*(n+1)/2) ).at n=8A202144
• Lucas-Carmichael numbers with 3 prime factors.at n=9A216925
• Numbers x such that sigma(x)=sigma(V(x)), where sigma(x) is the sum of the divisors of x and V(x) the transform defined in A245252.at n=8A245469
• a(n) = p(2*n)-p(2*n-2)-p(n) where p(n) are the partition numbers A000041(n).at n=21A263847
• a(n) is the smallest k different from n such that (n, k) is a Harshad amicable pair (see the comments).at n=28A272479
• Elliptic Carmichael numbers for the elliptic curve y^2 = x^3 + 80.at n=22A317174
• Odd composite integers m such that A005248(m) == 3 (mod m).at n=47A335672
• Odd composite integers m such that F(m)^2 == 1 (mod m), where F(m) is the m-th Fibonacci number.at n=31A337231
• a(n) is the least k such that n is divisible by the sum of digits of k and k is divisible by the sum of digits of n; a(n) = -1 if no such k exists.at n=28A344487