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In mathematics, in the area of numerical analysis, **Galerkin methods**, named for Russian mathematician Boris Galerkin, convert a continuous operator problem, such as a differential equation, commonly in a weak formulation, to a discrete problem by applying linear constraints determined by finite sets of basis functions.

Often when referring to a Galerkin method, one also gives the name along with typical assumptions and approximation methods used:

**Ritz–Galerkin method**(after Walther Ritz) typically assumes symmetric and positive definite bilinear form in the weak formulation, where the differential equation for a physical system can be formulated via minimization of a quadratic function representing the system energy and the approximate solution is a linear combination of the given set of the basis functions.^{[1]}**Bubnov–Galerkin method**(after Ivan Bubnov) does not require the bilinear form to be symmetric and substitutes the energy minimization with orthogonality constrains determined by the same basis functions that are used to approximate the solution. In an operator formulation of the differential equation, Bubnov–Galerkin method can be viewed as applying an orthogonal projection to the operator.**Petrov–Galerkin method**(after Georgii I. Petrov^{[2]}) allows using basis functions for orthogonality constrains (called**test basis functions**) that are different from the basis functions used to approximate the solution. Petrov–Galerkin method can be viewed as an extension of Bubnov–Galerkin method, applying an projection that is not necessarily orthogonal in the operator formulation of the differential equation.

Examples of Galerkin methods are:

- the Galerkin method of weighted residuals, the most common method of calculating the global stiffness matrix in the finite element method,
^{[3]}^{[4]} - the boundary element method for solving integral equations,
- Krylov subspace methods.
^{[5]}

Let us introduce Galerkin's method with an abstract problem posed as a weak formulation on a Hilbert space , namely,

- find such that for all .

Here, is a bilinear form (the exact requirements on will be specified later) and is a bounded linear functional on .

Choose a subspace of dimension *n* and solve the projected problem:

- Find such that for all .

We call this the **Galerkin equation**. Notice that the equation has remained unchanged and only the spaces have changed.
Reducing the problem to a finite-dimensional vector subspace allows us to numerically compute as a finite linear combination of the basis vectors in .

The key property of the Galerkin approach is that the error is orthogonal to the chosen subspaces. Since , we can use as a test vector in the original equation. Subtracting the two, we get the Galerkin orthogonality relation for the error, which is the error between the solution of the original problem, , and the solution of the Galerkin equation,

Since the aim of Galerkin's method is the production of a linear system of equations, we build its matrix form, which can be used to compute the solution algorithmically.

Let be a basis for . Then, it is sufficient to use these in turn for testing the Galerkin equation, i.e.: find such that

We expand with respect to this basis, and insert it into the equation above, to obtain

This previous equation is actually a linear system of equations , where

Due to the definition of the matrix entries, the matrix of the Galerkin equation is symmetric if and only if the bilinear form is symmetric.

Here, we will restrict ourselves to symmetric bilinear forms, that is

While this is not really a restriction of Galerkin methods, the application of the standard theory becomes much simpler. Furthermore, a Petrov–Galerkin method may be required in the nonsymmetric case.

The analysis of these methods proceeds in two steps. First, we will show that the Galerkin equation is a well-posed problem in the sense of Hadamard and therefore admits a unique solution. In the second step, we study the quality of approximation of the Galerkin solution .

The analysis will mostly rest on two properties of the bilinear form, namely

- Boundedness: for all holds
- for some constant

- Ellipticity: for all holds
- for some constant

By the Lax-Milgram theorem (see weak formulation), these two conditions imply well-posedness of the original problem in weak formulation. All norms in the following sections will be norms for which the above inequalities hold (these norms are often called an energy norm).

Since , boundedness and ellipticity of the bilinear form apply to . Therefore, the well-posedness of the Galerkin problem is actually inherited from the well-posedness of the original problem.

The error between the original and the Galerkin solution admits the estimate

This means, that up to the constant , the Galerkin solution is as close to the original solution as any other vector in . In particular, it will be sufficient to study approximation by spaces , completely forgetting about the equation being solved.

Since the proof is very simple and the basic principle behind all Galerkin methods, we include it here: by ellipticity and boundedness of the bilinear form (inequalities) and Galerkin orthogonality (equals sign in the middle), we have for arbitrary :

Dividing by and taking the infimum over all possible yields the lemma.

I. Elishakof, M. Amato, A. Marzani, P.A. Arvan, and J.N. Reddy ^{[6]}^{[7]}^{[8]}^{[9]} studied the application of the Galerkin method to stepped structures. They showed that the generalized function, namely unit-step function, Dirac’s delta function, and the doublet function are needed for obtaining accurate results.

The approach is usually credited to Boris Galerkin.^{[10]}^{[11]} The method was explained to the Western reader by Hencky^{[12]} and Duncan^{[13]}^{[14]} among others. Its convergence was studied by Mikhlin^{[15]} and Leipholz^{[16]}^{[17]}^{[18]}^{[19]} Its coincidence with Fourier method was illustrated by Elishakoff et al.^{[20]}^{[21]}^{[22]} Its equivalence to Ritz's method for conservative problems was shown by Singer.^{[23]} Gander and Wanner^{[24]} showed how Ritz and Galerkin methods led to the modern finite element method. One hundred years of method's development was discussed by Repin.^{[25]} Elishakoff, Kaplunov and Kaplunov^{[26]} show that the Galerkin’s method was not developed by Ritz, contrary to the Timoshenko’s statements.

**^**A. Ern, J.L. Guermond,*Theory and practice of finite elements*, Springer, 2004, ISBN 0-387-20574-8**^**"Georgii Ivanovich Petrov (on his 100th birthday)", Fluid Dynamics, May 2012, Volume 47, Issue 3, pp 289-291, DOI 10.1134/S0015462812030015**^**S. Brenner, R. L. Scott,*The Mathematical Theory of Finite Element Methods*, 2nd edition, Springer, 2005, ISBN 0-387-95451-1**^**P. G. Ciarlet,*The Finite Element Method for Elliptic Problems*, North-Holland, 1978, ISBN 0-444-85028-7**^**Y. Saad,*Iterative Methods for Sparse Linear Systems*, 2nd edition, SIAM, 2003, ISBN 0-89871-534-2**^**Elishakoff I., Marco Amato, Prakash Ankitha Arvan and Alessandro Marzani, 2021 “Rigorous implementation of the Galerkin method for stepped structures needs generalized functions,” Journal of Sound and Vibration, Vol. 490, article 115708**^**Elishakoff I., Marco Amato and Alessandro Marzani, 2021, “Galerkin's method revisited and corrected in the problem of Jaworski and Dowell”, Mechanical Systems and Signal Processing, Vol. 156, article 107604**^**Elishakoff I. and Marco Amato, 2021, “Flutter of a beam in supersonic flow: truncated version of Timoshenko–Ehrenfest equation is sufficient”, International Journal of Mechanics and Materials in Design, in press.**^**Marco Amato, Elishakoff I. and J. N. Reddy,2021, "Flutter of a multi-component beam in a supersonic flow", AIAA Journal, in press.**^**Galerkin, B.G.,1915, Rods and Plates, Series Occurring in Various Questions Concerning the Elastic Equilibrium of Rods and Plates, Vestnik Inzhenerov i Tekhnikov, (Engineers and Technologists Bulletin), Vol. 19, 897-908 (in Russian),(English Translation: 63-18925, Clearinghouse Fed. Sci. Tech. Info.1963).**^**"Le destin douloureux de Walther Ritz (1878-1909)", (Jean-Claude Pont, editor), Cahiers de Vallesia, 24, (2012), ISBN 978-2-9700636-5-0**^**Hencky H.,1927, Eine wichtige Vereinfachung der Methode von Ritz zur angennäherten Behandlung von Variationproblemen, ZAMM: Zeitschrift für angewandte Mathematik und Mechanik, Vol. 7, 80-81 (in German).**^**Duncan, W.J.,1937, Galerkin’s Method in Mechanics and Differential Equations, Aeronautical Research Committee Reports and Memoranda, No. 1798.**^**Duncan, W.J.,1938, The Principles of the Galerkin Method, Aeronautical Research Report and Memoranda, No. 1894.**^**S. G. Mikhlin, "Variational methods in Mathematical Physics", Pergamon Press, 1964**^**Leipholz H.H.E.,1976, Use of Galerkin’s Method for Vibration Problems, Shock and Vibration Digest, Vol. 8, 3-18**^**Leipholz H.H.E.,1967, Über die Wahl der Ansatzfunktionen bei der Durchfuchrung des Verfahrens von Galerkin, Acta Mech., Vol. 3, 295-317 (in German).**^**Leipholz H.H.E., 1967, Über die Befreiung der Anzatzfunktionen des Ritzschen und Galerkinschen Verfahrens von den Randbedingungen, Ing. Arch., Vol. 36, 251-261 (in German).**^**Leipholz, H.H.E.,1976, Use of Galerkin’s Method for Vibration Problems, The Shock and Vibration Digest Vol. 8, 3-18, 1976.**^**Elishakoff, I., Lee,L.H.N.,1986, On Equivalence of the Galerkin and Fourier Series Methods for One Class of Problems, Journal of Sound and Vibration, Vol. 109, 174-177.**^**Elishakoff, I., Zingales, M.,2003, Coincidence of Bubnov-Galerkin and Exact Solution in an Applied Mechanics Problem, Journal of Applied Mechanics, Vol. 70, 777-779.**^**Elishakoff, I., Zingales M.,2004, Convergence of Bubnov-Galerkin Method Exemplified, AIAA Journal, Vol. 42(9), 1931-1933.**^**Singer J.,1962, On Equivalence of the Galerkin and Rayleigh-Ritz Methods, Journal of the Royal Aeronautical Society, Vol. 66, No. 621, p.592.**^**Gander, M.J, Wanner, G.,2012, From Euler, Ritz, and Galerkin to Modern Computing, SIAM Review, Vol. 54(4), 627-666.**^**] Repin, S.,2017, One Hundred Years of the Galerkin Method, Computational Methods and Applied Mathematics, Vol.17(3), 351-357.**^**.Elishakoff, I., Julius Kaplunov, Elizabeth Kaplunov, 2020, “Galerkin’s method was not developed by Ritz, contrary to the Timoshenko’s statement”, in Nonlinear Dynamics of Discrete and Continuous Systems (A. Abramyan, I. Andrianov and V. Gaiko, eds.), pp. 63-82, Springer, Berlin.

- "Galerkin method",
*Encyclopedia of Mathematics*, EMS Press, 2001 [1994] - Galerkin Method from MathWorld