Bi-twin chain
In number theory, a bi-twin chain of length k + 1 is a sequence of natural numbers n − 1 , n + 1 , 2 n − 1 , 2 n + 1 , … , 2 k n − 1 , 2 k n + 1 {\displaystyle n-1,n+1,2n-1,2n+1,\dots ,2^{k}n-1,2^{k}n+1\,} in which every number is prime. The special case, when the four numbers n − 1 , n + 1 , 2 n − 1 , 2 n + 1 {\displaystyle n-1,n+1,2n-1,2n+1} are all primes, they are called bi-twin primes, such n values are 6, 30, 660, 810, 2130, 2550, 3330, 3390, 5850, 6270, 10530, 33180, 41610, 44130, 53550, 55440, 57330, 63840, 65100, 70380, 70980, 72270, 74100, 74760, 78780, 80670, 81930, 87540, 93240, … (sequence A066388 in the OEIS) Except 6, all of these numbers are divisible by 30.