A familiar use of modular arithmetic is its use in the 24-hour clock: the arithmetic of time-keeping in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23. If the time is 19:00 now — 7 o'clock in the evening — then 8 hours later it will be 3:00. Usual addition would suggest that the later time should be 19 + 8 = 27, but this is not the answer because clock time "wraps around" at the end of the day. Likewise, if the 24-hour clock starts at 12:00 (noon) and 21 hours elapse, then the time will be 09:00 the next day, rather than 33:00. Since the hour number starts over when it reaches 24, this is arithmetic modulo 24. Note: The clock shown below is not a 24-hour clock; it is the more widely used 12-hour, "modulo" 12, clock.
Two integers a and b are said to be congruent modulo n, if their difference a − b is an integer multiple of n. An equivalent definition is that both numbers have the same remainder when divided by n. If this is the case, it is expressed as:
The above mathematical statement is read: "a is congruent to b modulo n".
because 38 − 14 = 24, which is a multiple of 12. For positive n and non-negative a and b, congruence of a and b can also be thought of as asserting that these two numbers have the same remainder after dividing by the modulus n. So,
because both numbers, when divided by 12, have the same remainder (2). Equivalently, the fractional parts of doing a full division of each of the numbers by 12 are the same: .1666... (38/12 = 3.166..., 2/12 = .1666...). From the prior definition we also see that their difference, a - b = 36, is a whole number (integer) multiple of 12 (n = 12, 36/12 = 3).
The same rule holds for negative values of a:
A remark on the notation: Because it is common to consider several congruence relations for different moduli at the same time, the modulus is incorporated in the notation. In spite of the ternary notation, the congruence relation for a given modulus is binary. This would have been clearer if the notation a ≡n b had been used, instead of the common traditional notation.
The properties that make this relation a congruence relation (respecting addition, subtraction, and multiplication) are the following.
If and , then:
The set of congruence classes modulo n is denoted as (or, alternatively, or ) and defined by:
When n ≠ 0, has n elements, and can be written as:
When n = 0, does not have zero elements; rather, it is isomorphic to , since .
We can define addition, subtraction, and multiplication on by the following rules:
The verification that this is a proper definition uses the properties given before. In this way, becomes a commutative ring. For example, in the ring , we have
The set has a number of important mathematical properties that are foundational to various branches of mathematics.
Rather than excluding the special case n = 0, it is more useful to include (which, as mentioned before, is isomorphic to the ring of integers), for example when discussing the characteristic of a ring.
Parentheses are sometimes dropped from the expression, e.g. "38 ≡ 14 mod 12" or "2 = 14 mod 12", or placed around the divisor e.g. "38 ≡ 14 mod (12)". Notation such as "38(mod 12)" has also been observed, but is ambiguous without contextual clarification.
The congruence relation is sometimes expressed by using modulo instead of mod, like "38 ≡ 14 (modulo 12)" in computer science. The modulo function in various computer languages typically yield the common residue, for example the statement "y = MOD(38,12);" gives y = 2.
It is one of the foundations of number theory, touching on almost every aspect of its study, and provides key examples for group theory, ring theory and abstract algebra.
In cryptography, modular arithmetic directly underpins public key systems such as RSA and Diffie-Hellman, as well as providing finite fields which underlie elliptic curves, and is used in a variety of symmetric key algorithms including AES, IDEA, and RC4.
In computer science, modular arithmetic is often applied in bitwise operations and other operations involving fixed-width, cyclic data structures. The modulo operation, as implemented in many programming languages and calculators, is an application of modular arithmetic that is often used in this context.
In chemistry, the last digit of the CAS registry number (a number which is unique for each chemical compound) is a check digit, which is calculated by taking the last digit of the first two parts of the CAS registry number times 1, the next digit times 2, the next digit times 3 etc., adding all these up and computing the sum modulo 10.
In music, arithmetic modulo 12 is used in the consideration of the system of twelve-tone equal temperament, where octave and enharmonic equivalency occurs (that is, pitches in a 1∶2 or 2∶1 ratio are equivalent, and C-sharp is considered the same as D-flat).
The method of casting out nines offers a quick check of decimal arithmetic computations performed by hand. It is based on modular arithmetic modulo 9, and specifically on the crucial property that 10 ≡ 1 (mod 9).
More generally, modular arithmetic also has application in disciplines such as law (see e.g., apportionment), economics, (see e.g., game theory) and other areas of the social sciences, where proportional division and allocation of resources plays a central part of the analysis.
Some neurologists (see e.g., Oliver Sacks) theorize that so-called autistic savants utilize an "innate" modular arithmetic to compute such complex problems as what day of the week a distant date will fall on.
Solving a system of non-linear modular arithmetic equations is NP-complete. For details, see for example M. R. Garey, D. S. Johnson: Computers and Intractability, a Guide to the Theory of NP-Completeness, W. H. Freeman 1979.
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