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
Square root of 2 shown to contain no solutions as follows. Since formula_24 is even, formula_29 must be divisible by formula_30, which makes the entire equation congruent to modulo formula_30: The square of any odd number is always formula_33. The square of any even number is always formula_34. Since both formula_18 and formula_19 are odd and formula_24 is even, we have: which is impossible. Therefore, the fifth possibility is also ruled out, leaving the sixth to be the only possible combination to contain solutions, if any. An extension of this lemma is the result that two identical whole-number squares can never be added to