In mathematics, the symbolic method in invariant theory is an algorithm developed by Arthur Cayley,^[1] Siegfried Heinrich Aronhold,^[2] Alfred Clebsch,^[3] and Paul Gordan^[4] in the 19th century for computing invariants of algebraic forms. It is based on treating the form as if it were a power of a degree one form, which corresponds to embedding a symmetric power of a vector space into the symmetric elements of a tensor product of copies of it.

The symbolic method uses a compact, but rather confusing and mysterious notation for invariants, depending on the introduction of new symbols a, b, c, ... (from which the symbolic method gets its name) with apparently contradictory properties.

These symbols can be explained by the following example from Gordan.^[5] Suppose that

${\displaystyle \displaystyle f(x)=A_{0}x_{1}^{2}+2A_{1}x_{1}x_{2}+A_{2}x_{2}^{2}}$

is a binary quadratic form with an invariant given by the discriminant

${\displaystyle \displaystyle \Delta =A_{0}A_{2}-A_{1}^{2}.}$

The symbolic representation of the discriminant is

${\displaystyle \displaystyle 2\Delta =(ab)^{2}}$

where a and b are the symbols. The meaning of the expression (ab)² is as follows. First of all, (ab) is a shorthand form for the determinant of a matrix whose rows are a₁, a₂ and b₁, b₂, so

${\displaystyle \displaystyle (ab)=a_{1}b_{2}-a_{2}b_{1}.}$

Squaring this we get

${\displaystyle \displaystyle (ab)^{2}=a_{1}^{2}b_{2}^{2}-2a_{1}a_{2}b_{1}b_{2}+a_{2}^{2}b_{1}^{2}.}$

Next we pretend that

${\displaystyle \displaystyle f(x)=(a_{1}x_{1}+a_{2}x_{2})^{2}=(b_{1}x_{1}+b_{2}x_{2})^{2}}$

so that

${\displaystyle \displaystyle A_{i}=a_{1}^{2-i}a_{2}^{i}=b_{1}^{2-i}b_{2}^{i}}$

and we ignore the fact that this does not seem to make sense if f is not a power of a linear form. Substituting these values gives

${\displaystyle \displaystyle (ab)^{2}=A_{2}A_{0}-2A_{1}A_{1}+A_{0}A_{2}=2\Delta .}$

More generally if

${\displaystyle \displaystyle f(x)=A_{0}x_{1}^{n}+{\binom {n}{1}}A_{1}x_{1}^{n-1}x_{2}+\cdots +A_{n}x_{2}^{n}}$

is a binary form of higher degree, then one introduces new variables a₁, a₂, b₁, b₂, c₁, c₂, with the properties

${\displaystyle f(x)=(a_{1}x_{1}+a_{2}x_{2})^{n}=(b_{1}x_{1}+b_{2}x_{2})^{n}=(c_{1}x_{1}+c_{2}x_{2})^{n}=\cdots .}$

What this means is that the following two vector spaces are naturally isomorphic:

• The vector space of homogeneous polynomials in A₀,...A_n of degree m
• The vector space of polynomials in 2m variables a₁, a₂, b₁, b₂, c₁, c₂, ... that have degree n in each of the m pairs of variables (a₁, a₂), (b₁, b₂), (c₁, c₂), ... and are symmetric under permutations of the m symbols a, b, ....,

The isomorphism is given by mapping a^n−j
₁a^j
₂, b^n−j
₁b^j
₂, .... to A_j. This mapping does not preserve products of polynomials.

The extension to a form f in more than two variables x₁, x₂, x₃,... is similar: one introduces symbols a₁, a₂, a₃ and so on with the properties

${\displaystyle f(x)=(a_{1}x_{1}+a_{2}x_{2}+a_{3}x_{3}+\cdots )^{n}=(b_{1}x_{1}+b_{2}x_{2}+b_{3}x_{3}+\cdots )^{n}=(c_{1}x_{1}+c_{2}x_{2}+c_{3}x_{3}+\cdots )^{n}=\cdots .}$

The rather mysterious formalism of the symbolic method corresponds to embedding a symmetric product Sⁿ(V) of a vector space V into a tensor product of n copies of V, as the elements preserved by the action of the symmetric group. In fact this is done twice, because the invariants of degree n of a quantic of degree m are the invariant elements of SⁿS^m(V), which gets embedded into a tensor product of mn copies of V, as the elements invariant under a wreath product of the two symmetric groups. The brackets of the symbolic method are really invariant linear forms on this tensor product, which give invariants of SⁿS^m(V) by restriction.

• Umbral calculus

• Gordan, Paul (1987) [1887]. Kerschensteiner, Georg (ed.). Vorlesungen über Invariantentheorie (2nd ed.). New York York: AMS Chelsea Publishing. ISBN 9780828403283. MR 0917266.

Footnotes

• Dieudonné, Jean; Carrell, James B. (1970). "Invariant theory, old and new". Advances in Mathematics. 4: 1–80. doi:10.1016/0001-8708(70)90015-0. pp. 32–7, "Invariants of n-ary forms: the symbolic method. Reprinted as Dieudonné, Jean; Carrell, James B. (1971). Invariant theory, old and new. Academic Press. ISBN 0-12-215540-8.
• Dolgachev, Igor (2003). Lectures on invariant theory. London Mathematical Society Lecture Note Series. Vol. 296. Cambridge University Press. doi:10.1017/CBO9780511615436. ISBN 978-0-521-52548-0. MR 2004511. S2CID 118144995.
• Grace, John Hilton; Young, Alfred (1903), The Algebra of invariants, Cambridge University Press
• Hilbert, David (1993) [1897]. Theory of algebraic invariants. Cambridge University Press. ISBN 9780521444576. MR 1266168.
• Koh, Sebastian S., ed. (2009) [1987]. Invariant Theory. Lecture Notes in Mathematics. Vol. 1278. Springer. ISBN 9783540183600.
• Kung, Joseph P. S.; Rota, Gian-Carlo (1984). "The invariant theory of binary forms". Bulletin of the American Mathematical Society. New Series. 10 (1): 27–85. doi:10.1090/S0273-0979-1984-15188-7. ISSN 0002-9904. MR 0722856.