Source: Sudipta Das.
Nick McGrath sent:
We need numbers of the form n=a*10^D+b such that a is a divisor of n . n/a= 10^D + b/a Therefore a can take any value such that a is a divisor of b. b can take any value from 1 to 10^D-1. So, for example, when D=1. We have the complete set of n's defined by: b=1 a=1 i.e 11 b=2; a=1,2 12,22 b=3; a=1,3 13,31 b=4; a=1,2,4 14,24,44 b=5; a=1,5 15,55 b=6; a=1,2,3,6 16,26,36,66 b=7; a=1,7 17,77 b=8; a=1,2,4,8 18,28,48,88 b=9; a=1,3,9 19,39,99 for a total of 23. So, for each D the number of integers that exist is simply the total number of divisors of all the numbers from 1 to 10^D-1. Don't know how to do this except by writing a program but get 23, 473, 7053, 93643 for D=1,2,3,4 respectively.