The projective special linear group G = PSL(2,7) is a finite group in mathematics that has important applications in algebra, geometry, and number theory. It is the automorphism group of the Klein quartic, and it is the second-smallest nonabelian simple group, next to the alternating group A₅ = PSL(2,5).

Definition   Let SL(2,7) denote the group of all 2×2 matrices of determinant one over the finite field with 7 elements. Then G = PSL(2,7) is defined to be the quotient group SL(2,7) / {I,−I} obtained by identifying I and −I. In this article, we let G denote any group isomorphic to PSL(2,7).

G = PSL(2,7) has 168 elements. This can be seen by counting the possible columns; there are 7² − 1 = 48 possibilities for the first column, then 7² − 7 = 42 possibilities for the second column. We must divide by 7 − 1 = 6 to force the determinant equal to one, and then we must divide by 2 when we identify I and −I. The result is (48*42) / (6*2) = 168.

Simplicity of PSL(2,7)

It is a general result that PSL(n, q) is simple for n ≥ 2, q ≥ 2, unless (n, q) = (2,2) or (2,3). In the former case, PSL(n, q) is isomorphic to the symmetric group S₃, and in the latter case PSL(n, q) is isomorphic to alternating group A₄. In fact, PSL(2,7) is the second smallest nonabelian simple group, next to the alternating group A₅ = PSL(2,5).

Actions on projective spaces

G = PSL(2,7) acts via linear fractional transformation on the projective line P¹(7) over the field with 7 elements:

Every automorphism of P¹(7) arises in this way, and so G = PSL(2,7) can be thought of geometrically as the group of symmetries of the projective line P¹(7).

However, PSL(2,7) is also isomorphic to SL(3,2) (= GL(3,2)), the special (general) linear group of 3×3 matrices over the field with 2 elements. In a similar fashion, G = SL(3,2) acts on the projective Fano plane P²(2) over the field with 2 elements:

Again, every automorphism of P²(2) arises in this way, and so G = SL(3,2) can be thought of geometrically as the group of symmetries of the Fano plane.

Symmetries of the Klein quartic

The Klein quartic x³y + y³z + z³x = 0 is a Riemann surface, the only curve of genus 3 over the complex numbers C whose group of automorphisms is isomorphic to G. The order 168 of G is the maximum allowed for this genus. Klein's quartic pops up all over the place in mathematics, not least of which includes representation theory, homology theory, octonion multiplication, Fermat's last theorem, and Stark's theorem on imaginary quadratic number fields of class number 1!

External links

• The Klein Quartic in Number Theory (Noam Elkies)
• Bounded Complex Domains (Tony Smith)