int main()
    {
        ...
        metod(x0, v0, m, a, b)
        ...
        return 0;
    }

    struct Parametr
        {
            public double a;
            public double phi;
            public double omega;
        };

        Parametr func(float x0, float v0, float e)
        {
            Parametr p;
            double a0 = Math.Sqrt(x0*x0 + v0*v0);
            double phi0 = -Math.Atan(v0 / x0);
            double sin0 = Math.Sin(phi0), tan0 = Math.Tan(phi0), cos0 = Math.Cos(phi0), cos3 = Math.Cos(3*phi0);
            double phi1 = (1 / 32 * cos3 * tan0 - 3 / 8 * a0 * a0 * sin0 - 1 / 3 * Math.Sin(3 * phi0)) / (sin0 * tan0 + cos0);
            double a1 = a0 * (phi1 * tan0 - cos0 / (32 * cos0));
            double a = a0 + e * a1;
            double phi = phi0 + e * phi1;
            double omega = 1 + e * 3 * a * a / 8;
            p.a = a;
            p.phi = phi;
            p.omega = omega;
            return p;
        }

    void metod(float x0, float v0, float m, float a, float b)
        {
            float x, v;
            a /= m;
            b /= m;
            Parametr p;
            float e = b / a;
            for (float t = 0; t < 10; t = (float)(t + 0.1))
            {
                p = func(x0, v0, e);
                x = (float)(p.a * Math.Cos(p.omega * t + p.phi) + e * Math.Pow(p.a, 3) / 32 * Math.Cos(3 * (p.omega * t + p.phi)));
                v = (float)(p.a * p.omega * Math.Sin(p.omega * t + p.phi) + e * Math.Pow(p.a, 3)*p.omega / 32 * Math.Sin(3 * (p.omega * t + p.phi)));
            }
        }