The digital root (also repeated digital sum) of a natural number in a given radix is the (single digit) value obtained by an iterative process of summing digits, on each iteration using the result from the previous iteration to compute a digit sum. The process continues until a single-digit number is reached. For example, in base 10, the digital root of the number 12345 is 6 because the sum of the digits in the number is 1 + 2 + 3 + 4 + 5 = 15, then the addition process is repeated again for the resulting number 15, so that the sum of 1 + 5 equals 6, which is the digital root of that number. In base 10, this is equivalent to taking the remainder upon division by 9 (except when the digital root is 9, where the remainder upon division by 9 will be 0), which allows it to be used as a divisibility rule.
The digital root is the value modulo because and thus so regardless of position, the value is the same – – which is why digits can be meaningfully added. Concretely, for a three-digit number
To obtain the modular value with respect to other numbers , one can take weighted sums, where the weight on the -th digit corresponds to the value of modulo . In base 10, this is simplest for 2, 5, and 10, where higher digits vanish (since 2 and 5 divide 10), which corresponds to the familiar fact that the divisibility of a decimal number with respect to 2, 5, and 10 can be checked by the last digit (even numbers end in 0, 2, 4, 6, or 8).
Also of note is the modulus : since and thus taking the alternating sum of digits yields the value modulo .
It helps to see the digital root of a positive integer as the position it holds with respect to the largest multiple of less than the number itself. For example, in base 6 the digital root of 11 is 2, which means that 11 is the second number after . Likewise, in base 10 the digital root of 2035 is 1, which means that . If a number produces a digital root of exactly , then the number is a multiple of .
With this in mind the digital root of a positive integer may be defined by using floor function, as
The digital root of in base is the digital root of the sum of the digital root of and the digital root of . This property can be used as a sort of checksum, to check that a sum has been performed correctly.
The digital root of in base is congruent to the difference of the digital root of and the digital root of modulo .
The digital root of in base as follows:
The digital root of the product of nonzero single digit numbers in base is given by the Vedic Square in base .
The digital root of in base is the digital root of the product of the digital root of and the digital root of .
For example, the additive persistence of 2718 in base 10 is 2: first we find that 2 + 7 + 1 + 8 = 18, then that 1 + 8 = 9.
There is no limit to the additive persistence of a number in a number base . Proof: For a given number , the persistence of the number consisting of repetitions of the digit 1 is 1 higher than that of . The smallest numbers of additive persistence 0, 1, ... in base 10 are:
0, 10, 19, 199, 19 999 999 999 999 999 999 999, ... (sequence A006050 in the OEIS)
The next number in the sequence (the smallest number of additive persistence 5) is 2 × 102×(1022 − 1)/9 − 1 (that is, 1 followed by 2 222 222 222 222 222 222 222 nines). For any fixed base, the sum of the digits of a number is proportional to its logarithm; therefore, the additive persistence is proportional to the iterated logarithm.