Euler's totient function phi(n) is defined as the number of positive integers not exceeding n that are relatively prime to n, where 1 is counted as being relatively prime to all numbers.  So, for example, phi(20) = 8 because the eight integers 1, 3, 7, 9, 11, 13, 17, and 19 are relatively prime to 20.  The table below shows values of phi(n) for n less than or equal to 20.

n1234567891011121314151617181920
phi(n)112242646410412688166188

Euler's totient valence function v(n) is defined as the number of positive integers k such that phi(k) = n.  For instance, v(8) = 5 because only the five integers k = 15, 16, 20, 24, and 30 are such that phi(k) = 8.  The table below shows values of v(n) for n less than or equal to 16.  (For n not in the table, v(n) = 0.)

nv(n)k such that phi(k) = n
121, 2
233, 4, 6
445, 8, 10, 12
647, 9, 14, 18
8515, 16, 20, 24, 30
10211, 22
12613, 21, 26, 28, 36, 42
16617, 32, 34, 40, 48, 60

Evaluate v(21000).