In mathematics and physics, there are a large number of topics named in honor of Swiss mathematician Leonhard Euler (1707-1783), who made many important discoveries and innovations. Many of these items named after Euler include their own unique function, equation, formula, identity, number (single or sequence), or other mathematical entity. Many of these entities have been given simple and ambiguous names such as Euler's function, Euler's equation, and Euler's formula.

Euler's work touched upon so many fields that he is often the earliest written reference on a given matter. In an effort to avoid naming everything after Euler, some discoveries and theorems are attributed to the first person to have discovered them after Euler.

*Euler stamp-USSR-1957*

## Euler's conjectures

- Euler's conjecture (Waring's problem)
- Euler's sum of powers conjecture

## Euler's equations

*Usually, Euler's equation refers to one of (or a set of) differential equations (DEs). It is customary to classify them into ODEs and PDEs.*

*Otherwise, Euler's equation might refer to a non-differential equation, as in these three cases:*

- Euler–Lotka equation, a characteristic equation employed in mathematical demography
- Euler's pump and turbine equation
- Euler transform used to accelerate the convergence of an alternating series and is also frequently applied to the hypergeometric series

## Euler's ordinary differential equations

- Euler rotation equations, a set of first-order ODEs concerning the rotations of a rigid body.
- Euler–Cauchy equation, a linear equidimensional second-order ODEs with variable coefficients. Its second-order version can emerge from Laplace equation in polar coordinates.
- Euler–Bernoulli beam equation, a fourth-order ODE concerning the elasticity of structural beams.
- Euler–Lagrange equation, a second-order ODE emerging from minimization problems in calculus of variations.

## Euler's partial differential equations

- Euler conservation equations, a set of quasilinear first-order hyperbolic equations used in fluid dynamics for inviscid flows. In the (Froude) limit of no external field, they are conservation equations.
- Euler–Tricomi equation – a second-order PDE emerging from Euler conservation equations.
- Euler–Poisson–Darboux equation, a second-order PDE playing important role in solving the wave equation.

## Euler's formulas

- Euler's formula in complex analysis e
^{ ix}= cos x + i sin x - Euler's polyhedral formula for planar graphs or polyhedra: v − e + f = 2
- Euler's formula for the critical load of a column:

- Euler's continued fraction formula connecting a finite sum of products with a finite continued fraction
- Euler product formula for the Riemann zeta function.
- Euler–Maclaurin formula (Euler's summation formula) relating integrals to sums
- Euler–Rodrigues formula describing the rotation of a vector in three dimensions

## Euler's functions

- The Euler function, a modular form that is a prototypical q-series.
- Euler's homogeneous function theorem
- Euler's totient function (or Euler phi (φ) function) in number theory, counting the number of coprime integers less than an integer.
- Euler hypergeometric integral

## Euler's identities

- Euler's identity e
^{ iπ}+ 1 = 0. - Euler's four-square identity, which shows that the product of two sums of four squares can itself be expressed as the sum of four squares.
- Euler's identity may also refer to the pentagonal number theorem.

## Euler's numbers

- Euler's number, e ≈ 2.71828, the base of the natural logarithm, also known as Napier's constant.
- Euler's idoneal numbers, a set of 65 or possibly 66 integers with special properties
- Euler numbers are an integer sequence defined by the Taylor series expansion of the hyperbolic secant.
- Eulerian numbers count certain types of permutations.
- Euler number (physics), the cavitation number in fluid dynamics.
- Euler number (topology) – now, Euler characteristic, classically the number of vertices minus edges plus faces of a polyhedron.
- Lucky numbers of Euler
- Euler–Mascheroni constant, γ ≈ 0.5772, the limiting difference between the harmonic series and the natural logarithm.
- Eulerian integers are the numbers of form a + bω where ω is a complex cube root of 1.

## Euler's theorems

- Euler's homogeneous function theorem, a theorem about homogeneous polynomials.
- Euler's infinite tetration theorem
- Euler's rotation theorem
- Euler's theorem (differential geometry) on the existence of the principal curvatures of a surface and orthogonality of the associated principal directions.
- Euler's theorem in geometry, relating the circumcircle and incircle of a triangle.
- Euler's quadrilateral theorem, an extension of the parallogram law to convex quadrilaterals
- Euclid–Euler theorem, relating perfect numbers to Mersenne primes.
- Euler–Fermat theorem, that aφ(m) ≡ 1 (mod m) whenever a is coprime to m, and φ is Euler's totient function
- Euler's theorem equating the number of partitions with odd parts and the number of partitions with distinct parts. See Glaisher's theorem.
- Euler's adding-up theorem in economics

## Euler's laws

- Euler's first law, the linear momentum of a body is equal to the product of the mass of the body and the velocity of its center of mass.
- Euler's second law, the sum of the external moments about a point is equal to the rate of change of angular momentum about that point.

## Other things named after Euler

- 2002 Euler (a minor planet)
- AMS Euler typeface
- Euler (software)
- Euler acceleration or force
- Euler Book Prize
- Euler Medal, a prize for research in combinatorics
- Euler programming language
- Euler Society, an American group dedicated to the life and work of Leonhard Euler
- Euler–Fokker genus
- Project Euler
- Leonhard Euler Telescope
- Rue Euler (a street in Paris, France)

*Photo: NASA*

## Topics by field of study

*Selected topics from above, grouped by subject.*

**Analysis: derivatives, integrals, and logarithms**

- Euler approximation – (see Euler's method)
- Euler derivative (as opposed to Lagrangian derivative)
- The Euler integrals of the first and second kind, namely the beta function and gamma function.
- The Euler method, a method for finding numerical solutions of differential equations
- Semi-implicit Euler method
- Euler's number e ≈ 2.71828, the base of the natural logarithm, also known as Napier's constant.
- The Euler substitutions for integrals involving a square root.
- Euler's summation formula, a theorem about integrals.
- Cauchy–Euler equation (or Euler equation), a second-order linear differential equation
- Euler–Maclaurin formula – relation between integrals and sums
- Euler–Mascheroni constant or Euler's constant γ ≈ 0.577216

**Geometry and spatial arrangement**

- Euler angles defining a rotation in space.
- Euler brick
- Euler's line – relation between triangle centers
- Euler operator – set of functions to create polygon meshes
- Euler's rotation theorem
- Euler spiral – a curve whose curvature varies linearly with its arc length
- Euler squares, usually called Graeco-Latin squares.
- Euler's theorem in geometry, relating the circumcircle and incircle of a triangle.
- Euler's quadrilateral theorem, an extension of the parallogram law to convex quadrilaterals
- Euler–Rodrigues formulas concern Euler–Rodrigues parameters and 3D rotation matrices

**Graph theory**

- Euler characteristic (formerly called Euler number) in algebraic topology and topological graph theory, and the corresponding Euler's formula {\displaystyle \scriptstyle \chi (S^{2})=F-E+V=2} \scriptstyle \chi(S^2)=F-E+V=2
- Eulerian circuit, Euler cycle or Eulerian path – a path through a graph that takes each edge once
- Eulerian graph has all its vertices spanned by an Eulerian path
- Euler class
- Euler diagram – incorrectly, but more popularly, known as Venn diagrams, its subclass
- Euler tour technique

**Music**

- Euler–Fokker genus

**Number theory**

- Euler's criterion – quadratic residues modulo by primes
- Euler product – infinite product expansion, indexed by prime numbers of a Dirichlet series
- Euler pseudoprime
- Euler's totient function (or Euler phi (φ) function) in number theory, counting the number of coprime integers less than an integer.

**Physical systems**

- Euler's Disk – a toy consisting of a circular disk that spins, without slipping, on a surface
- Euler rotation equations, in rigid body dynamics.
- Euler conservation equations in fluid dynamics.
- Euler number (physics), the cavitation number in fluid dynamics.
- Euler's three-body problem
- Euler–Bernoulli beam equation, concerning the elasticity of structural beams.
- Euler formula in calculating the buckling load of columns.
- Euler–Tricomi equation – concerns transonic flow
- Euler integrals (thermodynamics) - Gives relationship between extensive variables in Thermodynamics

**Polynomials**

- Euler's homogeneous function theorem, a theorem about homogeneous polynomials.
- Euler polynomials
- Euler spline – composed of classical Euler polynomial arcs