153505domain: NAppears in sequencesNumber of steps to reach 1 in '3x+1' (or Collatz) problem starting with the n-th Mersenne prime.at n=22A181777Numbers m such that 72*m + 1, 576*m + 1, 648*m + 1, 1296*m + 1, and 2592*m + 1 are all primes.at n=31A372187