Modulo (jargon)

The word modulo (Latin, with respect to a modulus of ___) is the Latin ablative of modulus which itself means "a small measure." It was introduced into mathematics by Carl Friedrich Gauss in 1801. The term has gained many meanings—some exact and some imprecise. Generally, the term is expressed:

A is the same as B modulo C

means

A and B are the same except for differences accounted for or explained by C.

History

Modulo is mathematical jargon that was introduced into mathematics in the book Disquisitiones Arithmeticae by Carl Friedrich Gauss in 1801. Given the integers a, b and n, the expression a ≡ b (mod n) (pronounced "a is congruent to b modulo n") means that a − b is an integer multiple of n, or equivalently, a and b both leave the same remainder when divided by n. It is the Latin ablative of modulus, which itself means "a small measure."

The term has gained many meanings—some exact and some imprecise. The most general precise definition is simply in terms of an equivalence relation R, where a is equivalent or congruent to b modulo R if aRb. Generally, the term is expressed:

A is the same as B modulo C

means

A and B are the same except for differences accounted for or explained by C.

Usage

Original use

Gauss's original usage is, given the integers a, b and n, the expression a ≡ b (mod n) (pronounced "a is congruent to b modulo n") means that a − b is an integer multiple of n, or equivalently, a and b both leave the same remainder when divided by n. For example,

13 is congruent to 63 modulo 10

means

13 and 63 differ by a multiple of 10

Computing

In computing and computer science, the term is used:

  • In computing it is typically the modulo operation, given two numbers (either integer or real), a and n, a modulo n is the remainder after numerical division of a by n, under certain constraints.
  • "Operating modulo" is special jargon in category theory as applied to functional programming, in the sense of mapping a functor to a category by highlighting or defining remainders.[1]

Structures

The term can be used differently when referring to different mathematical structures:

  • Two members a and b of a group are congruent modulo a normal subgroup if and only if ab−1 is a member of the normal subgroup. See quotient group and isomorphism theorem.
  • Two members of a ring or an algebra are congruent modulo an ideal if the difference between them is in the ideal.
    • Used as a verb, the act of factoring out a normal subgroup (or an ideal) from a group (or ring) is often called "modding out the..." or "we now mod out the...".
  • Two subsets of an infinite set are equal modulo finite sets precisely if their symmetric difference is finite, that is, you can remove a finite piece from the first subset, then add a finite piece to it, and get as result the second subset.
  • A short exact sequence of maps leads to the definition of a quotient space as being one space modulo another; thus, for example, that a cohomology is the space of closed forms modulo exact forms.

Modding out

Generally, modding out is a somewhat informal term that means declaring things equivalent that otherwise would be considered distinct. For example, suppose the sequence 1 4 2 8 5 7 is to be regarded as the same as the sequence 7 1 4 2 8 5 because each is a cyclicly shifted version of the other:

In that case one is "modding out by cyclic shifts."

See also

References

  1. ↑ p. 22, Category Theory for Computing Science, Barr and Wells
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