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Mathlib.NumberTheory.LSeries.Convolution

Dirichlet convolution of sequences and products of L-series #

We define the Dirichlet convolution f ⍟ g of two sequences f g : ℕ → R with values in a semiring R by (f ⍟ g) n = ∑ (k * m = n), f k * g m when n ≠ 0 and (f ⍟ g) 0 = 0. Technically, this is done by transporting the existing definition for ArithmeticFunction R; see LSeries.convolution. We show that these definitions agree (LSeries.convolution_def).

We then consider the case R = ℂ and show that L (f ⍟ g) = L f * L g on the common domain of convergence of the L-series L f and L g of f and g; see LSeries_convolution and LSeries_convolution'.

Dirichlet convolution of two functions #

def toArithmeticFunction {R : Type u_1} [Zero R] (f : R) :

We turn any function ℕ → R into an ArithmeticFunction R by setting its value at 0 to be zero.

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    theorem toArithmeticFunction_congr {R : Type u_1} [Zero R] {f : R} {f' : R} (h : ∀ {n : }, n 0f n = f' n) :
    @[simp]

    If we consider an arithmetic function just as a function and turn it back into an arithmetic function, it is the same as before.

    noncomputable def LSeries.convolution {R : Type u_1} [Semiring R] (f : R) (g : R) :
    R

    Dirichlet convolution of two sequences.

    We define this in terms of the already existing definition for arithmetic functions.

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      Dirichlet convolution of two sequences.

      We define this in terms of the already existing definition for arithmetic functions.

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        theorem LSeries.convolution_congr {R : Type u_1} [Semiring R] {f : R} {f' : R} {g : R} {g' : R} (hf : ∀ {n : }, n 0f n = f' n) (hg : ∀ {n : }, n 0g n = g' n) :
        theorem ArithmeticFunction.coe_mul {R : Type u_1} [Semiring R] (f : ArithmeticFunction R) (g : ArithmeticFunction R) :
        LSeries.convolution f g = (f * g)

        The product of two arithmetic functions defines the same function as the Dirichlet convolution of the functions defined by them.

        theorem LSeries.convolution_def {R : Type u_1} [Semiring R] (f : R) (g : R) :
        LSeries.convolution f g = fun (n : ) => pn.divisorsAntidiagonal, f p.1 * g p.2
        @[simp]
        theorem LSeries.convolution_map_zero {R : Type u_1} [Semiring R] (f : R) (g : R) :

        Multiplication of L-series #

        theorem LSeries.term_convolution (f : ) (g : ) (s : ) (n : ) :
        LSeries.term (LSeries.convolution f g) s n = pn.divisorsAntidiagonal, LSeries.term f s p.1 * LSeries.term g s p.2

        We give an expression of the LSeries.term of the convolution of two functions in terms of a sum over Nat.divisorsAntidiagonal.

        theorem LSeries.term_convolution' (f : ) (g : ) (s : ) :
        LSeries.term (LSeries.convolution f g) s = fun (n : ) => ∑' (b : ((fun (p : × ) => p.1 * p.2) ⁻¹' {n})), LSeries.term f s (↑b).1 * LSeries.term g s (↑b).2

        We give an expression of the LSeries.term of the convolution of two functions in terms of an a priori infinite sum over all pairs (k, m) with k * m = n (the set we sum over is infinite when n = 0). This is the version needed for the proof that L (f ⍟ g) = L f * L g.

        theorem LSeriesHasSum.convolution {f : } {g : } {s : } {a : } {b : } (hf : LSeriesHasSum f s a) (hg : LSeriesHasSum g s b) :

        The L-series of the convolution product f ⍟ g of two sequences f and g equals the product of their L-series, assuming both L-series converge.

        theorem LSeries_convolution' {f : } {g : } {s : } (hf : LSeriesSummable f s) (hg : LSeriesSummable g s) :

        The L-series of the convolution product f ⍟ g of two sequences f and g equals the product of their L-series, assuming both L-series converge.

        theorem LSeries_convolution {f : } {g : } {s : } (hf : LSeries.abscissaOfAbsConv f < s.re) (hg : LSeries.abscissaOfAbsConv g < s.re) :

        The L-series of the convolution product f ⍟ g of two sequences f and g equals the product of their L-series in their common half-plane of absolute convergence.

        theorem LSeriesSummable.convolution {f : } {g : } {s : } (hf : LSeriesSummable f s) (hg : LSeriesSummable g s) :

        The L-series of the convolution product f ⍟ g of two sequences f and g is summable when both L-series are summable.

        Versions for arithmetic functions #

        theorem ArithmeticFunction.LSeriesHasSum_mul {f : ArithmeticFunction } {g : ArithmeticFunction } {s : } {a : } {b : } (hf : LSeriesHasSum (fun (n : ) => f n) s a) (hg : LSeriesHasSum (fun (n : ) => g n) s b) :
        LSeriesHasSum (fun (n : ) => (f * g) n) s (a * b)

        The L-series of the (convolution) product of two -valued arithmetic functions f and g equals the product of their L-series, assuming both L-series converge.

        theorem ArithmeticFunction.LSeries_mul' {f : ArithmeticFunction } {g : ArithmeticFunction } {s : } (hf : LSeriesSummable (fun (n : ) => f n) s) (hg : LSeriesSummable (fun (n : ) => g n) s) :
        LSeries (fun (n : ) => (f * g) n) s = LSeries (fun (n : ) => f n) s * LSeries (fun (n : ) => g n) s

        The L-series of the (convolution) product of two -valued arithmetic functions f and g equals the product of their L-series, assuming both L-series converge.

        theorem ArithmeticFunction.LSeries_mul {f : ArithmeticFunction } {g : ArithmeticFunction } {s : } (hf : (LSeries.abscissaOfAbsConv fun (n : ) => f n) < s.re) (hg : (LSeries.abscissaOfAbsConv fun (n : ) => g n) < s.re) :
        LSeries (fun (n : ) => (f * g) n) s = LSeries (fun (n : ) => f n) s * LSeries (fun (n : ) => g n) s

        The L-series of the (convolution) product of two -valued arithmetic functions f and g equals the product of their L-series in their common half-plane of absolute convergence.

        theorem ArithmeticFunction.LSeriesSummable_mul {f : ArithmeticFunction } {g : ArithmeticFunction } {s : } (hf : LSeriesSummable (fun (n : ) => f n) s) (hg : LSeriesSummable (fun (n : ) => g n) s) :
        LSeriesSummable (fun (n : ) => (f * g) n) s

        The L-series of the (convolution) product of two -valued arithmetic functions f and g is summable when both L-series are summable.