Many properties of a natural number
can be seen or directly computed from the prime factorization of
.
- The multiplicity of a prime factor
of
is the largest exponent
for which
divides
. The tables show the multiplicity for each prime factor. If no exponent is written then the multiplicity is
(since
). The multiplicity of a prime which does not divide
may be called
or may be considered undefined.
and
, the prime omega functions, count the number of prime factors of a natural number
.
(little omega) is the number of distinct prime factors of
.
(big omega) is the number of prime factors of
counted with multiplicity (so it is the sum of all prime factor multiplicities).
- A prime number has
. The first: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37 (sequence A000040 in the OEIS). There are many special types of prime numbers.
- A composite number has
. The first: 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21 (sequence A002808 in the OEIS). All numbers above 1 are either prime or composite. 1 is neither.
- A semiprime has
(so it is composite). The first: 4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34 (sequence A001358 in the OEIS).
- A
-almost prime (for a natural number
) has
(so it is composite if
).
- An even number has the prime factor 2. The first: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24 (sequence A005843 in the OEIS).
- An odd number does not have the prime factor 2. The first: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23 (sequence A005408 in the OEIS). All integers are either even or odd.
- A square has even multiplicity for all prime factors (it is of the form
for some
). The first: 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144 (sequence A000290 in the OEIS).
- A cube has all multiplicities divisible by 3 (it is of the form
for some
). The first: 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000, 1331, 1728 (sequence A000578 in the OEIS).
- A perfect power has a common divisor
for all multiplicities (it is of the form
for some
and
). The first: 4, 8, 9, 16, 25, 27, 32, 36, 49, 64, 81, 100 (sequence A001597 in the OEIS). 1 is sometimes included.
- A powerful number (also called squarefull) has multiplicity greater than 1 for all its prime factors. The first: 1, 4, 8, 9, 16, 25, 27, 32, 36, 49, 64, 72 (sequence A001694 in the OEIS).
- A prime power has only one prime factor, i.e.
. The first: 2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17, 19 (sequence A000961 in the OEIS). 1 is sometimes included.
- An Achilles number is powerful but not a perfect power. The first: 72, 108, 200, 288, 392, 432, 500, 648, 675, 800, 864, 968 (sequence A052486 in the OEIS).
- A square-free integer has no prime factor with multiplicity greater than 1. The first: 1, 2, 3, 5, 6, 7, 10, 11, 13, 14, 15, 17 (sequence A005117 in the OEIS). A number where some but not all prime factors have multiplicity greater than 1 is neither square-free nor squarefull, but squareful.
- The Liouville function
is 1 if
is even, and is -1 if
is odd.
- The Möbius function
is 0 if
is not square-free. Otherwise
is 1 if
is even, and is −1 if
is odd.
- A sphenic number is square-free and the product of 3 distinct primes, i.e. it has
. The first: 30, 42, 66, 70, 78, 102, 105, 110, 114, 130, 138, 154 (sequence A007304 in the OEIS).
, sometimes called the integer logarithm, is the sum of primes dividing
, counted with multiplicity. It is an additive function.
- A Ruth-Aaron pair is a pair of two consecutive numbers
with
. The first (by
value): 5, 8, 15, 77, 125, 714, 948, 1330, 1520, 1862, 2491, 3248 (sequence A039752 in the OEIS). Another definition is where the same prime is only counted once; if so, the first (by
value): 5, 24, 49, 77, 104, 153, 369, 492, 714, 1682, 2107, 2299 (sequence A006145 in the OEIS).
- A primorial
is the product of all primes from 2 to
. The first: 2, 6, 30, 210, 2310, 30030, 510510, 9699690, 223092870, 6469693230, 200560490130, 7420738134810 (sequence A002110 in the OEIS).
is sometimes included.
- A factorial
is the product of all numbers from 1 to
. The first: 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800, 39916800, 479001600 (sequence A000142 in the OEIS).
is sometimes included.
- A
-smooth number (for a natural number
) has its prime factors
(so it is also
-smooth for any
).
is smoother than
if the largest prime factor of
is less than the largest of
.
- A regular number has no prime factor greater than 5 (so it is 5-smooth). The first: 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16 (sequence A051037 in the OEIS).
- A
-powersmooth number has all
where
is a prime factor with multiplicity
.
- A frugal number has more digits than the number of digits in its prime factorization (when written like the tables below with multiplicities above 1 as exponents). The first in decimal: 125, 128, 243, 256, 343, 512, 625, 729, 1024, 1029, 1215, 1250 (sequence A046759 in the OEIS).
- An equidigital number has the same number of digits as its prime factorization. The first in decimal: 1, 2, 3, 5, 7, 10, 11, 13, 14, 15, 16, 17 (sequence A046758 in the OEIS).
- An extravagant number has fewer digits than its prime factorization. The first in decimal: 4, 6, 8, 9, 12, 18, 20, 22, 24, 26, 28, 30 (sequence A046760 in the OEIS).
- An economical number has been defined as a frugal number, but also as a number that is either frugal or equidigital.
(greatest common divisor of
and
) is the product of all prime factors which are both in
and
(with the smallest multiplicity for
and
).
and
are coprime (also called relatively prime) if they have no common prime factors, which implies
.
(least common multiple of
and
) is the product of all prime factors of
or
(with the largest multiplicity for
or
).
. Finding the prime factors is often harder than computing
and
using other algorithms which do not require known prime factorization.
is a divisor of
(also called
divides
, or
is divisible by
) if all prime factors of
have at least the same multiplicity in
.
- The divisors of
are all products of some or all prime factors of
(including the empty product 1 of no prime factors). The number of divisors can be computed by increasing all multiplicities by 1 and then multiplying them.
Divisors and properties related to divisors are shown in table of divisors.