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In mathematics, specifically in representation theory, a **semisimple representation** (also called a **completely reducible representation**) is a linear representation of a group or an algebra that is a direct sum of simple representations (also called irreducible representations).^{[1]} It is an example of the general mathematical notion of semisimplicity.

Many representations that appear in applications of representation theory are semisimple or can be approximated by semisimple representations. A semisimple module over an algebra over a field is an example of a semisimple representation. Conversely, a semisimple representation of a group *G* over a field k is a semisimple module over the group ring *k*[*G*].

Let *V* be a representation of a group *G*; or more generally, let *V* be a vector space with a set of linear endomorphisms acting on it. In general, a vector space acted on by a set of linear endomorphisms is said to be simple (or irreducible) if the only invariant subspaces for those operators are zero and the vector space itself; a semisimple representation then is a direct sum of simple representations in that sense.^{[1]}

The following are equivalent:^{[2]}

*V*is semisimple as a representation.*V*is a sum of simple subrepresentations.- Each subrepresentation
*W*of*V*admits a complementary representation: a subrepresentation*W'*such that .

The equivalences of the above conditions can be shown based on the next lemma, which is of independent interest:

**Lemma ^{[3]}** — Let

*Proof of the lemma*: Write where are simple representations. Without loss of generality, we can assume are subrepresentations; i.e., we can assume the direct sum is internal. Now, consider the family of all possible direct sums with various subsets . Put the partial ordering on it by saying the direct sum over *K* is less than the direct sum over *J* if . By Zorn's lemma, we can find a maximal such that . We claim that . By definition, so we only need to show that . If is a proper subrepresentatiom of then there exists such that . Since is simple (irreducible), . This contradicts the maximality of , so as claimed. Hence, is a section of *p*.

Note that we cannot take to the set of such that . The reason is that it can happen, and frequently does, that is a subspace of and yet . For example, take , and to be three distinct lines through the origin in . For an explicit counterexample, let be the algebra of matrices and set , the regular representation of . Set and and set . Then , and are all irreducible -modules and . Let be the natural surjection. Then and . In this case, but because this sum is not direct.

*Proof of equivalences*^{[4]} : Take *p* to be the natural surjection . Since *V* is semisimple, *p* splits and so, through a section, is isomorphic to a subrepretation that is complementary to *W*.

: We shall first observe that every nonzero subrepresentation *W* has a simple subrepresentation. Shrinking *W* to a (nonzero) cyclic subrepresentation we can assume it is finitely generated. Then it has a maximal subrepresentation *U*. By the condition 3., for some . By modular law, it implies . Then is a simple subrepresentation
of *W* ("simple" because of maximality). This establishes the observation. Now, take to be the sum of all simple subrepresentations, which, by 3., admits a complementary representation . If , then, by the early observation, contains a simple subrepresentation and so , a nonsense. Hence, .

:^{[5]} The implication is a direct generalization of a basic fact in linear algebra that a basis can be extracted from a spanning set of a vector space. That is we can prove the following slightly more precise statement:

- When is a sum of simple subrepresentations, a semisimple decomposition , some subset , can be extracted from the sum.

As in the proof of the lemma, we can find a maximal direct sum that consists of some ’s. Now, for each *i* in *I*, by simplicity, either or . In the second case, the direct sum is a contradiction to the maximality of *W*. Hence, .

A finite-dimensional unitary representation (i.e., a representation factoring through a unitary group) is a basic example of a semisimple representation. Such a representation is semisimple since if *W* is a subrepresentation, then the orthogonal complement to *W* is a complementary representation^{[6]} because if and , then for any *w* in *W* since *W* is *G*-invariant, and so .

For example, given a continuous finite-dimensional complex representation of a finite group or a compact group *G*, by the averaging argument, one can define an inner product on *V* that is *G*-invariant: i.e., , which is to say is a unitary operator and so is a unitary representation.^{[6]} Hence, every finite-dimensional continuous complex representation of *G* is semisimple.^{[7]} For a finite group, this is a special case of Maschke's theorem, which says a finite-dimensional representation of a finite group *G* over a field k with characteristic not dividing the order of *G* is semisimple.^{[8]}^{[9]}

By Weyl's theorem on complete reducibility, every finite-dimensional representation of a semisimple Lie algebra over a field of characteristic zero is semisimple.^{[10]}

Given a linear endomorphism *T* of a vector space *V*, *V* is semisimple as a representation of *T* (i.e., *T* is a semisimple operator) if and only if the minimal polynomial of *T* is separable; i.e., a product of distinct irreducible polynomials.^{[11]}

Given a finite-dimensional representation *V*, the Jordan–Hölder theorem says there is a filtration by subrepresentations: such that each successive quotient is a simple representation. Then the associated vector space is a semisimple representation called an **associated semisimple representation**, which, up to an isomorphism, is uniquely determined by *V*.^{[12]}

A representation of a unipotent group is generally not semisimple. Take to be the group consisting of real matrices ; it acts on in a natural way and makes *V* a representation of *G*. If *W* is a subrepresentation of *V* that has dimension 1, then a simple calculation shows that it must be spanned by the vector . That is, there are exactly three *G*-subrepresentations of *V*; in particular, *V* is not semisimple (as a unique one-dimensional subrepresentation does not admit a complementary representation).^{[13]}

The decomposition of a semisimple representation into simple ones, called a semisimple decomposition, need not be unique; for example, for a trivial representation, simple representations are one-dimensional vector spaces and thus a semisimple decomposition amounts to a choice of a basis of the representation vector space.^{[14]} The isotypic decomposition, on the other hand, is an example of a unique decomposition.^{[15]}

However, for a finite-dimensional semisimple representation *V* over an algebraically closed field, the numbers of simple representations up to isomorphisms appearing in the decomposition of *V* (1) are unique and (2) completely determine the representation up to isomorphisms;^{[16]} this is a consequence of Schur's lemma in the following way. Suppose a finite-dimensional semisimple representation *V* over an algebraically closed field is given: by definition, it is a direct sum of simple representations. By grouping together simple representations in the decomposition that are isomorphic to each other, up to an isomorphism, one finds a decomposition (not necessarily unique):^{[16]}

where are simple representations, mutually non-isomorphic to one another, and are positive integers. By Schur's lemma,

- ,

where refers to the equivariant linear maps. Also, each is unchanged if is replaced by another simple representation isomorphic to . Thus, the integers are independent of chosen decompositions; they are the *multiplicities* of simple representations , up to isomorphisms, in *V*.^{[17]}

In general, given a finite-dimensional representation of a group *G* over a field *k*, the composition is called the character of .^{[18]} When is semisimple with the decomposition as above, the trace is the sum of the traces of with multiplicities and thus, as functions on *G*,

where are the characters of . When *G* is a finite group or more generally a compact group and is a unitary representation with the inner product given by the averaging argument, the Schur orthogonality relations say:^{[19]} the irreducible characters (characters of simple representations) of *G* are an orthonormal subset of the space of complex-valued functions on *G* and thus .

There is a decomposition of a semisimple representation that is unique, called *the* isotypic decomposition of the representation. By definition, given a simple representation *S*, the isotypic component of type *S* of a representation *V* is the sum of all subrepresentations of *V* that are isomorphic to *S*;^{[15]} note the component is also isomorphic to the direct sum of some choice of subrepresentations isomorphic to *S* (so the component is unique, while the summands are not necessary so).

Then the isotypic decomposition of a semisimple representation *V* is the (unique) direct sum decomposition:^{[15]}^{[20]}

where is the set of isomorphism classes of simple representations of *G* and is the isotypic component of *V* of type *S* for some .

Let be the space of homogeneous degree-three polynomials over the complex numbers in variables . Then acts on by permutation of the three variables. This is a finite-dimensional complex representation of a finite group, and so is semisimple. Therefore, this 10-dimensional representation can be broken up into three isotypic components, each corresponding to one of the three irreducible representations of . In particular, contains three copies of the trivial representation, one copy of the sign representation, and three copies of the two-dimensional irreducible representation of . For example, the span of and is isomorphic to . This can more easily be seen by writing this two-dimensional subspace as

- .

Another copy of can be written in a similar form:

- .

So can the third:

- .

Then is the isotypic component of type in .

In Fourier analysis, one decomposes a (nice) function as the *limit* of the Fourier series of the function. In much the same way, a representation itself may not be semisimple but it may be the completion (in a suitable sense) of a semisimple representation. The most basic case of this is the Peter–Weyl theorem, which decomposes the left (or right) regular representation of a compact group into the Hilbert-space completion of the direct sum of all simple unitary representations. As a corollary,^{[21]} there is a natural decomposition for = the Hilbert space of (classes of) square-integrable functions on a compact group *G*:

where means the completion of the direct sum and the direct sum runs over all isomorphism classes of simple finite-dimensional unitary representations of *G*.^{[note 1]} Note here that every simple unitary representation (up to an isomorphism) appears in the sum with the multiplicity the dimension of the representation.

When the group *G* is a finite group, the vector space is simply the group algebra of *G* and also the completion is vacuous. Thus, the theorem simply says that

That is, each simple representation of *G* appears in the regular representation with multiplicity the dimension of the representation.^{[22]} This is one of standard facts in the representation theory of a finite group (and is much easier to prove).

When the group *G* is the circle group , the theorem exactly amounts to the classical Fourier analysis.^{[23]}

In quantum mechanics and particle physics, the angular momentum of an object can be described by complex representations of the rotation group|SO(3), all of which are semisimple.^{[24]} Due to connection between SO(3) and SU(2), the non-relativistic spin of an elementary particle is described by complex representations of SU(2) and the relativistic spin is described by complex representations of SL_{2}(**C**), all of which are semisimple.^{[24]} In angular momentum coupling, Clebsch–Gordan coefficients arise from the multiplicities of irreducible representations occurring in the semisimple decomposition of a tensor product of irreducible representations.^{[25]}

**^**To be precise, the theorem concerns the regular representation of and the above statement is a corollary.

- ^
^{a}^{b}Procesi 2007, Ch. 6, § 1.1, Definition 1 (ii). **^**Procesi 2007, Ch. 6, § 2.1.**^**Anderson & Fuller 1992, Proposition 9.4.**^**Anderson & Fuller 1992, Theorem 9.6.**^**Anderson & Fuller 1992, Lemma 9.2.- ^
^{a}^{b}Fulton & Harris 1991, § 9.3. A **^**Hall 2015, Theorem 4.28**^**Fulton & Harris 1991, Corollary 1.6.**^**Serre 1977, Theorem 2.**^**Hall 2015 Theorem 10.9**^**Jacobson 1989, § 3.5. Exercise 4.**^**Artin 1999, Ch. V, § 14.**^**Fulton & Harris 1991, just after Corollary 1.6.**^**Serre 1977, § 1.4. remark- ^
^{a}^{b}^{c}Procesi 2007, Ch. 6, § 2.3. - ^
^{a}^{b}Fulton & Harris 1991, Proposition 1.8. **^**Fulton & Harris 1991, § 2.3.**^**Fulton & Harris 1991, § 2.1. Definition**^**Serre 1977, § 2.3. Theorem 3 and § 4.3.**^**Serre 1977, § 2.6. Theorem 8 (i)**^**Procesi 2007, Ch. 8, Theorem 3.2.**^**Serre 1977, § 2.4. Corollary 1 to Proposition 5**^**Procesi 2007, Ch. 8, § 3.3.- ^
^{a}^{b}Hall, Brian C. (2013). "Angular Momentum and Spin".*Quantum Theory for Mathematicians*. Graduate Texts in Mathematics. Vol. 267. Springer. pp. 367–392. ISBN 978-1461471158. **^**Klimyk, A. U.; Gavrilik, A. M. (1979). "Representation matrix elements and Clebsch–Gordan coefficients of the semisimple Lie groups".*Journal of Mathematical Physics*.**20**(1624): 1624–1642. Bibcode:1979JMP....20.1624K. doi:10.1063/1.524268.

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