Mathematical constants

#include <cmath>
#include <iomanip>
#include <iostream>
#include <limits>
#include <numbers>
#include <string_view>
 
auto egamma_aprox(const unsigned iterations)
{
 long double s{};
 for (unsigned m{2}; m != iterations; ++m)
 if (const long double t{std::riemann_zetal (m) / m}; m % 2)
 s -= t;
 else
 s += t;
 return s;
};
 
int main()
{
 using namespace std::numbers;
 using namespace std::string_view_literals;
 
 const auto x = std::sqrt (inv_pi) / inv_sqrtpi +
 std::ceil (std::exp2 (log2e)) + sqrt3 * inv_sqrt3 + std::exp (0);
 const auto v = (phi * phi - phi) + 1 / std::log2 (sqrt2) +
 log10e * ln10 + std::pow (e, ln2) - std::cos (pi); 
 std::cout << "The answer is " << x * v << '\n';
 
 constexpr auto γ{"0.577215664901532860606512090082402"sv};
 std::cout
 << "γ as 106 sums of ±ζ(m)/m = "
 << egamma_aprox(1'000'000) << '\n'
 << "γ as egamma_v<float> = "
 << std::setprecision (std::numeric_limits <float>::digits10 + 1)
 << egamma_v<float> << '\n'
 << "γ as egamma_v<double> = "
 << std::setprecision (std::numeric_limits <double>::digits10 + 1)
 << egamma_v<double> << '\n'
 << "γ as egamma_v<long double> = "
 << std::setprecision (std::numeric_limits <long double>::digits10 + 1)
 << egamma_v<long double> << '\n'
 << "γ with " << γ.length() - 1 << " digits precision = " << γ << '\n';
}

The answer is 42
γ as 106 sums of ±ζ(m)/m = 0.577215
γ as egamma_v<float> = 0.5772157
γ as egamma_v<double> = 0.5772156649015329
γ as egamma_v<long double> = 0.5772156649015328606
γ with 34 digits precision = 0.577215664901532860606512090082402