42121

Appears in sequences

• Strong pseudoprimes to base 24.at n=16A020250
• Strong pseudoprimes to base 38.at n=26A020264
• Strong pseudoprimes to base 54.at n=21A020280
• Numbers n such that phi(n) = 4*phi(n-1).at n=5A268126
• Numbers k such that 3^(k-1) == 2^(k-1) !== 1 (mod k).at n=36A285300
• Numbers k such that -3 is a quadratic residue (not necessarily coprime) modulo k, k + 1, k + 2 and k + 3.at n=22A318527
• Composite numbers k of the form 4u+1 for which the odd part of phi(k) divides k-1.at n=23A339870
• Least number k such that the concatenation of n consecutive numbers from k to k+n-1 is divisible by their sum.at n=39A375728