#### Keith Number Checker

P
```using System;

namespace Algorithms.Numeric;

/// <summary>
///  In number theory, a Keith number or repfigit number is a natural number n in a given number base b with k digits such that
///  when a sequence is created such that the first k terms are the k digits of n and each subsequent term is the sum of the
///  previous k terms, n is part of the sequence.
/// </summary>
public static class KeithNumberChecker
{
/// <summary>
///     Checks if a number is a Keith number or not.
/// </summary>
/// <param name="number">Number to check.</param>
/// <returns>True if it is a Keith number; False otherwise.</returns>
public static bool IsKeithNumber(int number)
{
if (number < 0)
{
throw new ArgumentException(\$"{nameof(number)} cannot be negative");
}

var tempNumber = number;

var stringNumber = number.ToString();

var digitsInNumber = stringNumber.Length;

/* storing the terms of the series */
var termsArray = new int[number];

for (var i = digitsInNumber - 1; i >= 0; i--)
{
termsArray[i] = tempNumber % 10;
tempNumber /= 10;
}

var sum = 0;
var k = digitsInNumber;
while (sum < number)
{
sum = 0;

for (var j = 1; j <= digitsInNumber; j++)
{
sum += termsArray[k - j];
}

termsArray[k] = sum;
k++;
}

return sum == number;
}
}
```