# Euclid & Number Theory

The static Euclid class in the MathNet.Numerics namespace provides routines related to the domain of integers.

## Remainder vs. Canonical Modulus

Remainder and modulus are closely related operations with a long tradition of confusing on with the other. The % operator in most computer languages implements one of the two, but some even leave which one as an implementation detail (e.g. C-1990).

Warning: In C#, like most languages, % is the remainder operator, not the modulus!

#### Remainder

The remainder is the amount left over after performing the division of a dividend by a divisor, $$\frac{dividend}{divisor}$$, which do not divide evenly, that is, where the result of the division cannot be expressed as an integer. It is thus natural that the remainder has the sign of the dividend.

In C# and F#, the remainder is available as % operator, in VB as Mod. Alternatively you can use the Reminder function:

 1: 2: 3: 4:  Euclid.Remainder( 5, 3); // = 2, such that 5 = 1*3 + 2 Euclid.Remainder(-5, 3); // = -2, such that -5 = -1*3 - 2 Euclid.Remainder( 5, -3); // = 2, such that 5 = -1*-3 + 2 Euclid.Remainder(-5, -3); // = -2, such that -5 = 1*-3 - 2 

#### Modulus

On the other hand, in modular arithmetic numbers "wrap around" upon reaching a certain value n, or when crossing zero. Two real numbers are said to be congruent modulo n when their difference is an integer multiple of n. The modulo operator normalizes the dividend to the fundamental or smallest values congruent modulo n, where n is the divisor, and thus to the interval from 0 to n (including 0 but excluding n, possibly negative). It is thus natural that the modulus always has the sign of the divisor.

 1: 2: 3: 4:  Euclid.Modulus( 5, 3); // = 2, congruent modulo 3 by 5 - 1*3 Euclid.Modulus(-5, 3); // = 1, congruent modulo 3 by -5 + 2*3 Euclid.Modulus( 5, -3); // = -1, congruent modulo -3 by 5 + 2*-3 Euclid.Modulus(-5, -3); // = -2, congruent modulo -3 by -5 - 1*-3 

A typical case where the modulus appears in daily life is when grouping students into 3 groups by letting them line up and count through as 0 1 2 0 1 2 0 1 2 etc. This way, each student will end up in the group of their order within the line modulus 3.

## Integer Properties

#### Even or Odd?

Very simple question yet still somewhat error-prone to implement such that it works correctly for both positive and negative integers: is a number even or odd?

• IsEven(number)
• IsOdd(number)

#### Powers of two and Squares

Powers of two are prevalent in computer engineering. For performance reasons it is often preferable to align data in blocks where the size is a power of two, i.e. $$2^k$$. The CeilingToPowerOfTwo function helps in such situations by finding the smallest perfect power of two larger than or equal to the provided argument. There is also IsPowerOfTwo to determine whether a number is such a power of two, and PowerOfTwo to compute it efficiently.

When switching the operands of $$2^k$$ we get the square $$k^2$$. IsPerfectSquare determines whether the integer argument is a perfect square, i.e. a square of an integer.

## Euclid's Algorithm

#### Greatest Common Divisor

The GreatestCommonDivisor evaluates the GCD of either two integers or a full list or array of them using Euclid's algorithm. An extended version also returns how exactly the GCD can be composed from two integer arguments.

 1: 2: 3: 4: 5:  Euclid.GreatestCommonDivisor(10, 15, 45); // 5 long x, y; Euclid.ExtendedGreatestCommonDivisor(45, 18, out x, out y) // 9 // -> x=1, y=-2, hence 9 == 1*45 + -2*18 

#### Least Common Multiple

Closely related to the GCD, LeastCommonMultiple returns the LCM of two or more integers.

 1:  Euclid.LeastCommonMultiple(3, 5, 6); // 30