The definition of Euler's `phi` function is that `phi(n)` equals the number of positive integers less than or equal to `n` that are coprime, or relatively prime to `n`. Coprimes have a greatest common denominator of 1.
Euler's product formula gives that `n` satisfies
`phi(n) = n prod_(p_n)(1-1/p)`
so we have that, since `phi(n) = 10`,
`n prod_(p_n)(1-1/p) = 10`
where the product is taken over prime factors of `n`.
The smallest `n` that gives `phi(n) = 10` is `n=11`. This can be seen using the product formula:
`phi(11) = 11 prod_(p_n)(1-1/p) = 11 (1-1/11) = 11(10/11) = 10`
Since 11 is prime, 11 is the only prime factor of 11. The 10 coprimes of 11 are
The smallest integer satisfying phi(n) = 10 is n = 11