eulers_totient_function.cpp

/**
 * @file
 * @brief C++ Program to find
 * [Euler's Totient](https://en.wikipedia.org/wiki/Euler%27s_totient_function)
 * function
 *
 * Euler Totient Function is also known as phi function.
 * \f[\phi(n) =
 * \phi\left({p_1}^{a_1}\right)\cdot\phi\left({p_2}^{a_2}\right)\ldots\f] where
 * \f$p_1\f$, \f$p_2\f$, \f$\ldots\f$ are prime factors of n.
 * <br/>3 Euler's properties:
 * 1. \f$\phi(n) = n-1\f$
 * 2. \f$\phi(n^k) = n^k - n^{k-1}\f$
 * 3. \f$\phi(a,b) = \phi(a)\cdot\phi(b)\f$ where a and b are relative primes.
 *
 * Applying this 3 properties on the first equation.
 * \f[\phi(n) =
 * n\cdot\left(1-\frac{1}{p_1}\right)\cdot\left(1-\frac{1}{p_2}\right)\cdots\f]
 * where \f$p_1\f$,\f$p_2\f$... are prime factors.
 * Hence Implementation in \f$O\left(\sqrt{n}\right)\f$.
 * <br/>Some known values are:
 * * \f$\phi(100) = 40\f$
 * * \f$\phi(1) = 1\f$
 * * \f$\phi(17501) = 15120\f$
 * * \f$\phi(1420) = 560\f$
 */
#include <cstdlib>
#include <iostream>

/** Function to caculate Euler's totient phi
 */
uint64_t phiFunction(uint64_t n) {
    uint64_t result = n;
    for (uint64_t i = 2; i * i <= n; i++) {
        if (n % i == 0) {
            while (n % i == 0) {
                n /= i;
            }
            result -= result / i;
        }
    }
    if (n > 1)
        result -= result / n;
    return result;
}

/// Main function
int main(int argc, char *argv[]) {
    uint64_t n;
    if (argc < 2) {
        std::cout << "Enter the number: ";
    } else {
        n = strtoull(argv[1], nullptr, 10);
    }
    std::cin >> n;
    std::cout << phiFunction(n);
    return 0;
}