The number of digits 
in an integer 
is the number of numbers in some base (usually 10) required
to represent it. The numbers 1 to 9 are therefore single digits, while the numbers
10 to 99 are double digits. Terms such as "double-digit inflation" are
occasionally encountered, although this particular usage has thankfully not been
needed in the U.S. for some time. The number of base-
digits in a number 
can be calculated as
 |
(1)
|
where 
is the floor
function. For 
,
the formula becomes
 |
(2)
|
The number of digits 
in the number 
represented in base 
is given by the Wolfram Language function
DigitCount[n,
b, d], with DigitCount[n,
b] giving a list of the numbers of each digit in 
. The total number of digits in a number is given by IntegerLength[n,
b].
The positive integers with distinct base-10 digits are given by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 23, ... (OEIS A010784).
The number of 
-digit
integers is given by
 |  |  |
(3)
|
 |  |  |
(4)
|
 |  |  |
(5)
|
 |  |  |
(6)
|
where 
is a Pochhammer
symbol. For 
,
2, ..., the first few values are 9, 81, 648, 4536, 27216, 136080, 544320, 1632960,
3265920, and 3265920 (OEIS A073531). There
are therefore exactly
 |  |  |
(7)
|
 |  |  |
(8)
|
such numbers (Pondiczery 1975-a pseudonym for Ralph P. Boas; Foregger 1976), the largest of which is 9876543210.
The sums of the reciprocals of these 8877690 integers (Pondiczery 1975, Foregger 1976) is a rational number with numerator having 14816583 digits and denominator having 14816582 digits and given by
 |  |  |
(9)
|
 |  |  |
(10)
|
(OEIS A117914), computed by E. W. Weisstein on Apr. 1, 2006 using gridMathematica.
Numbers in base-10 which are divisible by their digits are 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 15, 22, 24, 33, 36, 44, 48, 55, 66, 77, 88, 99, 111, 112, 115, 122, ... (OEIS A034838). Numbers which are divisible by the sum of their digits are called Harshad numbers: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 18, 20, 21, 24, ... (OEIS A005349). Numbers which are divisible by both their digits and the sum of their digits are 1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 24, 36, 48, 111, 112, 126, 132, 135, 144, ... (OEIS A050104). Numbers which are equal to (i.e., not just divisible by) the product of their divisors and the sum of their divisors are called sum-product numbers and are given by 1, 135, 144, ... (OEIS A038369).
 | order | OEIS | Numbers ( ) |
| 2 | increasing | ||
| 2 | nondecreasing | A000225 | 3, 7, 15, 31, 63, 127, 255, 511, 1023, ... |
| 2 | nonincreasing | A023758 | 2, 3, 4, 6, 7, 8, 12, 14, 15, 16, 24, 28, 30, 31, ... |
| 2 | decreasing | 2 | |
| 10 | increasing | A009993 | 12, 13, 14, 15, 16, 17, 18, 19, 23, 24, 25, 26, ... |
| 10 | nondecreasing | A009994 | 11, 12, 13, 14, 15, 16, 17, 18, 19, 22, 23, 24, ... |
| 10 | nonincreasing | A009996 | 10, 11, 20, 21, 22, 30, 31, 32, 33, 40, 41, 42, ... |
| 10 | decreasing | A009995 | 10, 20, 21, 30, 31, 32, 40, 41, 42, 43, 50, 51, ... |
| 16 | increasing | A023784 | 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, ... |
| 16 | nondecreasing | A023757 | 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, ... |
| 16 | nonincreasing | A023771 | 17, 32, 33, 34, 48, 49, 50, 51, 64, 65, 66, 67, ... |
| 16 | decreasing | A023797 | 32, 33, 48, 49, 50, 64, 65, 66, 67, 80, 81, 82, ... |
In hexadecimal, numbers with increasing digits are called metadromes, those with nondecreasing digits are called plaindrones, those with nonincreasing digits are called nialpdromes, and those with decreasing digits are called katadromes.
The count of numbers with strictly increasing digits in base-
is 
,
and the number with strictly decreasing digits is 
.
