# [ITRIX12E] R Numbers

December 06, 2017

R-numbers are numbers which have the property that they do not have the digit ‘0 ’ and sum of every two adjacent digits of the number is prime. 123 is a R-number because 1+2 =3 and 2+3 =5 and 3 , 5 are primes.

How many R-numbers can be formed with at most length N?