13th root 13th root of a 100-digit number. All world records for mentally extracting a 13th root have been for numbers with an integer root. 13th root Extracting the 13th root of a number is a famous category for the mental calculation world records. The challenge consists of being given a large number (possibly over 100 digits) and asked to return the number that, when taken to the 13th power, equals the given number. For example, the 13th root of 8,192 is 2 and the 13th root of 96,889,010,407 is 7. Extracting the 13th root has certain properties. One is that the
Multiplicative digital root of 9876 is 0 and its multiplicative persistence (the number of steps required to reach a single digit) is 2. Multiplicative digital root The multiplicative digital root of a positive integer "n" is found by multiplying the digits of "n" together, then repeating this operation until only a single digit remains. This single-digit number is called the multiplicative digital root of "n". Multiplicative digital roots depend upon the base in which "n" is written. If the term is used without qualification, it is assumed that "n" is written in base 10. Multiplicative digital roots are the multiplicative equivalent of digital