Sum of digit

                     The digit sum of a number, say 152, is just the sum of the digits, 1+5+2=8. If the sum of the digits is greater than nine then the process is repeated. For example, the sum of the digits for 786 is 7+8+6=21 and the sum of the digits for 21 is 3 so the digit sum of 786 is 3. In some laces this concept of the digit sum of a number is called its reduced digit sum, but that terminology becomes too cumbersome. As in the preceding material the sum of the digit of a number may be different from its digit sum. The digit sum is the end result of repeatedly computing the sum of the digits until a single digit answer is obtained. The digit sum of a number n is denoted as DigitSum(n).