(* Title: HOL/Computational_Algebra/Polynomial_FPS.thy Author: Manuel Eberl, TU München *) section ‹Converting polynomials to formal power series› theory Polynomial_FPS imports Polynomial Formal_Power_Series begin context includes fps_notation begin definition fps_of_poly where "fps_of_poly p = Abs_fps (coeff p)" lemma fps_of_poly_eq_iff: "fps_of_poly p = fps_of_poly q ⟷ p = q" by (simp add: fps_of_poly_def poly_eq_iff fps_eq_iff) lemma fps_of_poly_nth [simp]: "fps_of_poly p $ n = coeff p n" by (simp add: fps_of_poly_def) lemma fps_of_poly_const: "fps_of_poly [:c:] = fps_const c" proof (subst fps_eq_iff, clarify) fix n :: nat show "fps_of_poly [:c:] $ n = fps_const c $ n" by (cases n) (auto simp: fps_of_poly_def) qed lemma fps_of_poly_0 [simp]: "fps_of_poly 0 = 0" by (subst fps_const_0_eq_0 [symmetric], subst fps_of_poly_const [symmetric]) simp lemma fps_of_poly_1 [simp]: "fps_of_poly 1 = 1" by (simp add: fps_eq_iff) lemma fps_of_poly_1' [simp]: "fps_of_poly [:1:] = 1" by (subst fps_const_1_eq_1 [symmetric], subst fps_of_poly_const [symmetric]) (simp add: one_poly_def) lemma fps_of_poly_numeral [simp]: "fps_of_poly (numeral n) = numeral n" by (simp add: numeral_fps_const fps_of_poly_const [symmetric] numeral_poly) lemma fps_of_poly_numeral' [simp]: "fps_of_poly [:numeral n:] = numeral n" by (simp add: numeral_fps_const fps_of_poly_const [symmetric] numeral_poly) lemma fps_of_poly_fps_X [simp]: "fps_of_poly [:0, 1:] = fps_X" by (auto simp add: fps_of_poly_def fps_eq_iff coeff_pCons split: nat.split) lemma fps_of_poly_add: "fps_of_poly (p + q) = fps_of_poly p + fps_of_poly q" by (simp add: fps_of_poly_def plus_poly.rep_eq fps_plus_def) lemma fps_of_poly_diff: "fps_of_poly (p - q) = fps_of_poly p - fps_of_poly q" by (simp add: fps_of_poly_def minus_poly.rep_eq fps_minus_def) lemma fps_of_poly_uminus: "fps_of_poly (-p) = -fps_of_poly p" by (simp add: fps_of_poly_def uminus_poly.rep_eq fps_uminus_def) lemma fps_of_poly_mult: "fps_of_poly (p * q) = fps_of_poly p * fps_of_poly q" by (simp add: fps_of_poly_def fps_times_def fps_eq_iff coeff_mult atLeast0AtMost) lemma fps_of_poly_smult: "fps_of_poly (smult c p) = fps_const c * fps_of_poly p" using fps_of_poly_mult[of "[:c:]" p] by (simp add: fps_of_poly_mult fps_of_poly_const) lemma fps_of_poly_sum: "fps_of_poly (sum f A) = sum (λx. fps_of_poly (f x)) A" by (cases "finite A", induction rule: finite_induct) (simp_all add: fps_of_poly_add) lemma fps_of_poly_sum_list: "fps_of_poly (sum_list xs) = sum_list (map fps_of_poly xs)" by (induction xs) (simp_all add: fps_of_poly_add) lemma fps_of_poly_prod: "fps_of_poly (prod f A) = prod (λx. fps_of_poly (f x)) A" by (cases "finite A", induction rule: finite_induct) (simp_all add: fps_of_poly_mult) lemma fps_of_poly_prod_list: "fps_of_poly (prod_list xs) = prod_list (map fps_of_poly xs)" by (induction xs) (simp_all add: fps_of_poly_mult) lemma fps_of_poly_pCons: "fps_of_poly (pCons (c :: 'a :: semiring_1) p) = fps_const c + fps_of_poly p * fps_X" by (subst fps_mult_fps_X_commute [symmetric], intro fps_ext) (auto simp: fps_of_poly_def coeff_pCons split: nat.split) lemma fps_of_poly_pderiv: "fps_of_poly (pderiv p) = fps_deriv (fps_of_poly p)" by (intro fps_ext) (simp add: fps_of_poly_nth coeff_pderiv) lemma fps_of_poly_power: "fps_of_poly (p ^ n) = fps_of_poly p ^ n" by (induction n) (simp_all add: fps_of_poly_mult) lemma fps_of_poly_monom: "fps_of_poly (monom (c :: 'a :: comm_ring_1) n) = fps_const c * fps_X ^ n" by (intro fps_ext) simp_all lemma fps_of_poly_monom': "fps_of_poly (monom (1 :: 'a :: comm_ring_1) n) = fps_X ^ n" by (simp add: fps_of_poly_monom) lemma fps_of_poly_div: assumes "(q :: 'a :: field poly) dvd p" shows "fps_of_poly (p div q) = fps_of_poly p / fps_of_poly q" proof (cases "q = 0") case False from False fps_of_poly_eq_iff[of q 0] have nz: "fps_of_poly q ≠ 0" by simp from assms have "p = (p div q) * q" by simp also have "fps_of_poly … = fps_of_poly (p div q) * fps_of_poly q" by (simp add: fps_of_poly_mult) also from nz have "… / fps_of_poly q = fps_of_poly (p div q)" by (intro nonzero_mult_div_cancel_right) (auto simp: fps_of_poly_0) finally show ?thesis .. qed simp lemma fps_of_poly_divide_numeral: "fps_of_poly (smult (inverse (numeral c :: 'a :: field)) p) = fps_of_poly p / numeral c" proof - have "smult (inverse (numeral c)) p = [:inverse (numeral c):] * p" by simp also have "fps_of_poly … = fps_of_poly p / numeral c" by (subst fps_of_poly_mult) (simp add: numeral_fps_const fps_of_poly_pCons) finally show ?thesis by simp qed lemma subdegree_fps_of_poly: assumes "p ≠ 0" defines "n ≡ Polynomial.order 0 p" shows "subdegree (fps_of_poly p) = n" proof (rule subdegreeI) from assms have "monom 1 n dvd p" by (simp add: monom_1_dvd_iff) thus zero: "fps_of_poly p $ i = 0" if "i < n" for i using that by (simp add: monom_1_dvd_iff') from assms have "¬monom 1 (Suc n) dvd p" by (auto simp: monom_1_dvd_iff simp del: power_Suc) then obtain k where k: "k ≤ n" "fps_of_poly p $ k ≠ 0" by (auto simp: monom_1_dvd_iff' less_Suc_eq_le) with zero[of k] have "k = n" by linarith with k show "fps_of_poly p $ n ≠ 0" by simp qed lemma fps_of_poly_dvd: assumes "p dvd q" shows "fps_of_poly (p :: 'a :: field poly) dvd fps_of_poly q" proof (cases "p = 0 ∨ q = 0") case False with assms fps_of_poly_eq_iff[of p 0] fps_of_poly_eq_iff[of q 0] show ?thesis by (auto simp: fps_dvd_iff subdegree_fps_of_poly dvd_imp_order_le) qed (insert assms, auto) lemmas fps_of_poly_simps = fps_of_poly_0 fps_of_poly_1 fps_of_poly_numeral fps_of_poly_const fps_of_poly_fps_X fps_of_poly_add fps_of_poly_diff fps_of_poly_uminus fps_of_poly_mult fps_of_poly_smult fps_of_poly_sum fps_of_poly_sum_list fps_of_poly_prod fps_of_poly_prod_list fps_of_poly_pCons fps_of_poly_pderiv fps_of_poly_power fps_of_poly_monom fps_of_poly_divide_numeral lemma fps_of_poly_pcompose: assumes "coeff q 0 = (0 :: 'a :: idom)" shows "fps_of_poly (pcompose p q) = fps_compose (fps_of_poly p) (fps_of_poly q)" using assms by (induction p rule: pCons_induct) (auto simp: pcompose_pCons fps_of_poly_simps fps_of_poly_pCons fps_compose_add_distrib fps_compose_mult_distrib) lemmas reify_fps_atom = fps_of_poly_0 fps_of_poly_1' fps_of_poly_numeral' fps_of_poly_const fps_of_poly_fps_X text ‹ The following simproc can reduce the equality of two polynomial FPSs two equality of the respective polynomials. A polynomial FPS is one that only has finitely many non-zero coefficients and can therefore be written as @{term "fps_of_poly p"} for some polynomial ‹p›. This may sound trivial, but it covers a number of annoying side conditions like @{term "1 + fps_X ≠ 0"} that would otherwise not be solved automatically. › ML ‹ (* TODO: Support for division *) signature POLY_FPS = sig val reify_conv : conv val eq_conv : conv val eq_simproc : cterm -> thm option end structure Poly_Fps = struct fun const_binop_conv s conv ct = case Thm.term_of ct of (Const (s', _) $ _ $ _) => if s = s' then Conv.binop_conv conv ct else raise CTERM ("const_binop_conv", [ct]) | _ => raise CTERM ("const_binop_conv", [ct]) fun reify_conv ct = let val rewr = Conv.rewrs_conv o map (fn thm => thm RS @{thm eq_reflection}) val un = Conv.arg_conv reify_conv val bin = Conv.binop_conv reify_conv in case Thm.term_of ct of (Const (@{const_name "fps_of_poly"}, _) $ _) => ct |> Conv.all_conv | (Const (@{const_name "Groups.plus"}, _) $ _ $ _) => ct |> ( bin then_conv rewr @{thms fps_of_poly_add [symmetric]}) | (Const (@{const_name "Groups.uminus"}, _) $ _) => ct |> ( un then_conv rewr @{thms fps_of_poly_uminus [symmetric]}) | (Const (@{const_name "Groups.minus"}, _) $ _ $ _) => ct |> ( bin then_conv rewr @{thms fps_of_poly_diff [symmetric]}) | (Const (@{const_name "Groups.times"}, _) $ _ $ _) => ct |> ( bin then_conv rewr @{thms fps_of_poly_mult [symmetric]}) | (Const (@{const_name "Rings.divide"}, _) $ _ $ (Const (@{const_name "Num.numeral"}, _) $ _)) => ct |> (Conv.fun_conv (Conv.arg_conv reify_conv) then_conv rewr @{thms fps_of_poly_divide_numeral [symmetric]}) | (Const (@{const_name "Power.power"}, _) $ Const (@{const_name "fps_X"},_) $ _) => ct |> ( rewr @{thms fps_of_poly_monom' [symmetric]}) | (Const (@{const_name "Power.power"}, _) $ _ $ _) => ct |> ( Conv.fun_conv (Conv.arg_conv reify_conv) then_conv rewr @{thms fps_of_poly_power [symmetric]}) | _ => ct |> ( rewr @{thms reify_fps_atom [symmetric]}) end fun eq_conv ct = case Thm.term_of ct of (Const (@{const_name "HOL.eq"}, _) $ _ $ _) => ct |> ( Conv.binop_conv reify_conv then_conv Conv.rewr_conv @{thm fps_of_poly_eq_iff[THEN eq_reflection]}) | _ => raise CTERM ("poly_fps_eq_conv", [ct]) val eq_simproc = try eq_conv end › simproc_setup poly_fps_eq ("(f :: 'a fps) = g") = ‹K (K Poly_Fps.eq_simproc)› lemma fps_of_poly_linear: "fps_of_poly [:a,1 :: 'a :: field:] = fps_X + fps_const a" by simp lemma fps_of_poly_linear': "fps_of_poly [:1,a :: 'a :: field:] = 1 + fps_const a * fps_X" by simp lemma fps_of_poly_cutoff [simp]: "fps_of_poly (poly_cutoff n p) = fps_cutoff n (fps_of_poly p)" by (simp add: fps_eq_iff coeff_poly_cutoff) lemma fps_of_poly_shift [simp]: "fps_of_poly (poly_shift n p) = fps_shift n (fps_of_poly p)" by (simp add: fps_eq_iff coeff_poly_shift) definition poly_subdegree :: "'a::zero poly ⇒ nat" where "poly_subdegree p = subdegree (fps_of_poly p)" lemma coeff_less_poly_subdegree: "k < poly_subdegree p ⟹ coeff p k = 0" unfolding poly_subdegree_def using nth_less_subdegree_zero[of k "fps_of_poly p"] by simp (* TODO: Move ? *) definition prefix_length :: "('a ⇒ bool) ⇒ 'a list ⇒ nat" where "prefix_length P xs = length (takeWhile P xs)" primrec prefix_length_aux :: "('a ⇒ bool) ⇒ nat ⇒ 'a list ⇒ nat" where "prefix_length_aux P acc [] = acc" | "prefix_length_aux P acc (x#xs) = (if P x then prefix_length_aux P (Suc acc) xs else acc)" lemma prefix_length_aux_correct: "prefix_length_aux P acc xs = prefix_length P xs + acc" by (induction xs arbitrary: acc) (simp_all add: prefix_length_def) lemma prefix_length_code [code]: "prefix_length P xs = prefix_length_aux P 0 xs" by (simp add: prefix_length_aux_correct) lemma prefix_length_le_length: "prefix_length P xs ≤ length xs" by (induction xs) (simp_all add: prefix_length_def) lemma prefix_length_less_length: "(∃x∈set xs. ¬P x) ⟹ prefix_length P xs < length xs" by (induction xs) (simp_all add: prefix_length_def) lemma nth_prefix_length: "(∃x∈set xs. ¬P x) ⟹ ¬P (xs ! prefix_length P xs)" by (induction xs) (simp_all add: prefix_length_def) lemma nth_less_prefix_length: "n < prefix_length P xs ⟹ P (xs ! n)" by (induction xs arbitrary: n) (auto simp: prefix_length_def nth_Cons split: if_splits nat.splits) (* END TODO *) lemma poly_subdegree_code [code]: "poly_subdegree p = prefix_length ((=) 0) (coeffs p)" proof (cases "p = 0") case False note [simp] = this define n where "n = prefix_length ((=) 0) (coeffs p)" from False have "∃k. coeff p k ≠ 0" by (auto simp: poly_eq_iff) hence ex: "∃x∈set (coeffs p). x ≠ 0" by (auto simp: coeffs_def) hence n_less: "n < length (coeffs p)" and nonzero: "coeffs p ! n ≠ 0" unfolding n_def by (auto intro!: prefix_length_less_length nth_prefix_length) show ?thesis unfolding poly_subdegree_def proof (intro subdegreeI) from n_less have "fps_of_poly p $ n = coeffs p ! n" by (subst coeffs_nth) (simp_all add: degree_eq_length_coeffs) with nonzero show "fps_of_poly p $ prefix_length ((=) 0) (coeffs p) ≠ 0" unfolding n_def by simp next fix k assume A: "k < prefix_length ((=) 0) (coeffs p)" also have "… ≤ length (coeffs p)" by (rule prefix_length_le_length) finally show "fps_of_poly p $ k = 0" using nth_less_prefix_length[OF A] by (simp add: coeffs_nth degree_eq_length_coeffs) qed qed (simp_all add: poly_subdegree_def prefix_length_def) end end