# mathlibdocumentation

ring_theory.trace

# Trace for (finite) ring extensions. #

Suppose we have an R-algebra S with a finite basis. For each s : S, the trace of the linear map given by multiplying by s gives information about the roots of the minimal polynomial of s over R.

## Implementation notes #

Typically, the trace is defined specifically for finite field extensions. The definition is as general as possible and the assumption that we have fields or that the extension is finite is added to the lemmas as needed.

We only define the trace for left multiplication (algebra.left_mul_matrix, i.e. algebra.lmul_left). For now, the definitions assume S is commutative, so the choice doesn't matter anyway.

## References #

@[simp]
theorem algebra.trace_apply (R : Type u_1) (S : Type u_2) [comm_ring R] [comm_ring S] [ S] (ᾰ : S) :
S) = S) ( S) ᾰ)
def algebra.trace (R : Type u_1) (S : Type u_2) [comm_ring R] [comm_ring S] [ S] :

The trace of an element s of an R-algebra is the trace of (*) s, as an R-linear map.

Equations
theorem algebra.trace_eq_zero_of_not_exists_basis (R : Type u_1) {S : Type u_2} [comm_ring R] [comm_ring S] [ S] (h : ¬∃ (s : set S) (b : R S), s.finite) :
= 0
theorem algebra.trace_eq_matrix_trace {R : Type u_1} {S : Type u_2} [comm_ring R] [comm_ring S] [ S] {ι : Type w} [fintype ι] [decidable_eq ι] (b : R S) (s : S) :
S) s = R R) s)
theorem algebra.trace_algebra_map_of_basis {R : Type u_1} {S : Type u_2} [comm_ring R] [comm_ring S] [ S] {ι : Type w} [fintype ι] (b : R S) (x : R) :
S) ( S) x) =

If x is in the base field K, then the trace is [L : K] * x.

@[simp]
theorem algebra.trace_algebra_map {K : Type u_4} {L : Type u_5} [field K] [field L] [ L] (x : K) :
L) ( L) x) =

If x is in the base field K, then the trace is [L : K] * x.

(If L is not finite-dimensional over K, then trace and finrank return 0.)

def algebra.trace_form (R : Type u_1) (S : Type u_2) [comm_ring R] [comm_ring S] [ S] :
S

The trace_form maps x y : S to the trace of x * y. It is a symmetric bilinear form and is nondegenerate if the extension is separable.

Equations
@[simp]
theorem algebra.trace_form_apply (R : Type u_1) {S : Type u_2} [comm_ring R] [comm_ring S] [ S] (x y : S) :
S) x y = S) (x * y)
theorem algebra.trace_form_is_sym (R : Type u_1) {S : Type u_2} [comm_ring R] [comm_ring S] [ S] :
theorem algebra.trace_form_to_matrix (R : Type u_1) {S : Type u_2} [comm_ring R] [comm_ring S] [ S] {ι : Type w} [fintype ι] (b : R S) [decidable_eq ι] (i j : ι) :
S) i j = S) ((b i) * b j)
theorem algebra.trace_form_to_matrix_power_basis (R : Type u_1) {S : Type u_2} [comm_ring R] [comm_ring S] [ S] (h : S) :
S) = λ (i j : fin h.dim), S) (h.gen ^ (i + j))