using System.Collections.Generic;

using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var tests = new[]{6, -15, 0};

        foreach (var test in tests)

        {

            Console.WriteLine(ShowSequence(test));

        }

    }

​

    public static string ShowSequence(int n)

    {

        if (n == 0)

        {

            return n + " = 0";

        }

​

        if (n < 0)

        {

            return n + " < 0";

        }

​

        // generate all numbers from 0 to n. 

        var enumeration = Enumerable.Range(0, n+1);

        var plusOperatorConcatenation = String.Join(" + ",  enumeration ) ;

        return plusOperatorConcatenation + " = " + enumeration.Sum();

    }

}