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In modular arithmetic, the modular multiplicative inverse of a is also defined: it is the number x such that ax ≡ 1 (mod n). This multiplicative inverse exists if and only if a and n are coprime. For example, the inverse of 3 modulo 11 is 4 because 4 ⋅ 3 ≡ 1 (mod 11). The extended Euclidean algorithm may be used to compute it.
A modular multiplicative inverse of a modulo m can be found by using the extended Euclidean algorithm. The Euclidean algorithm determines the greatest common divisor (gcd) of two integers, say a and m. If a has a multiplicative inverse modulo m, this gcd must be 1. The last of several equations produced by the algorithm may be solved for this gcd.
4.1 Powers of odd primes. ... and the inverse by complex inversion ... A Dirichlet character is a completely multiplicative function : ...
For any element x in a ring R, one has x0 = 0 = 0x (zero is an absorbing element with respect to multiplication) and (–1)x = –x. If 0 = 1 in a ring R (or more generally, 0 is a unit element), then R has only one element, and is called the zero ring. If a ring R contains the zero ring as a subring, then R itself is the zero ring. [6]
The following properties of the Dirichlet inverse hold: [4] The function f has a Dirichlet inverse if and only if f(1) ≠ 0. The Dirichlet inverse of a multiplicative function is again multiplicative. The Dirichlet inverse of a Dirichlet convolution is the convolution of the inverses of each function: () =.
The Dirichlet series that generates the Möbius function is the (multiplicative) inverse of the Riemann zeta function; if is a complex number with real part larger than 1 we have ∑ n = 1 ∞ μ ( n ) n s = 1 ζ ( s ) . {\displaystyle \sum _{n=1}^{\infty }{\frac {\mu (n)}{n^{s}}}={\frac {1}{\zeta (s)}}.}
The product and the multiplicative inverse of two roots of unity are also roots of unity. In fact, if x m = 1 and y n = 1, then (x −1) m = 1, and (xy) k = 1, where k is the least common multiple of m and n. Therefore, the roots of unity form an abelian group under multiplication. This group is the torsion subgroup of the circle group.
The multiplicative inverse x ≡ a −1 (mod m) may be efficiently computed by solving Bézout's equation a x + m y = 1 for x, y, by using the Extended Euclidean algorithm. In particular, if p is a prime number, then a is coprime with p for every a such that 0 < a < p; thus a multiplicative inverse exists for all a that is not congruent to zero ...