N-th member of sequence

using System;
using System.Collections.Generic;

class MainClass
{
    public static void Main()
    {
        Console.Write("Enter n = ");
        int n = int.Parse(Console.ReadLine());
        Console.WriteLine(Solve(n));
        Console.WriteLine(SolveRecursevely(n));
        Console.WriteLine(SolveMemoization(n));
    }

    public static double Solve(int n)
    {
        double n1 = 4 / 3.0;
        double n2 = 3;
        double n3 = 4;
        if (n == 1) return n1;
        if (n == 2) return n2;
        if (n == 3) return n3;
        double nth = 0;
        for (int i = 4; i <= n; i++)
        {
            nth = n3 + 2 * n2 - 8 * n1;
            n1 = n2;
            n2 = n3;
            n3 = nth;
        }
        return nth;
    }

    public static double SolveRecursevely(int n)
    {
        if (n == 1) return 4 / 3.0;
        if (n == 2) return 3;
        if (n == 3) return 4;
        return SolveRecursevely(n - 1) + 2 * SolveRecursevely(n - 2) - 8 * SolveRecursevely(n - 3);
    }

    public static double SolveMemoization(int n)
    {
        double result;
        if (memo.ContainsKey(n))
        {
            result = memo[n];
        }
        else
        {
            result = SolveMemoization(n - 1) + 2 * SolveMemoization(n - 2) - 8 * SolveMemoization(n - 3);
            memo[n] = result;
        }
        return result;

    }

    static Dictionary<int, double> memo = new Dictionary<int, double>() {
            {1, 4 / 3.0},
            {2, 3},
            {3, 4}
        };
}