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介绍De Bruijn定理的母函数形式,由此《(组合数学笔记)Pólya计数理论》系列完结。
与Pólya定理的母函数形式的推导类似,首先引入模式清单 ( I n v ) (\mathrm{Inv}) (Inv)的概念,并介绍两个引理。
设方案集 C X C^X CX关于幂群 H G H^G HG的模式清单 I n v ( C X / H G ) \mathrm{Inv}(C^X/H^G) Inv(CX/HG),并假定轨道集
C X / H G = { O r b ( φ 1 ) , O r b ( φ 2 ) , ⋯ , O r b ( φ k ) } , C^X/H^G=\big\{\mathrm{Orb}(\varphi_1),\,\mathrm{Orb}(\varphi_2),\,\cdots,\,\mathrm{Orb}(\varphi_k)\big\}, CX/HG={ Orb(φ1),Orb(φ2),⋯,Orb(φk)},
那么有
I n v ( C X / H G ) = ∑ O r b ( φ ) ∈ C X / H G w ( φ ) = ∑ j = 1 k w ( φ j ) . \mathrm{Inv}\big(C^X/H^G\big)=\sum_{\mathrm{Orb}(\varphi)\in C^X/H^G} w(\varphi)=\sum_{j=1}^k w(\varphi_j). Inv(CX/HG)=Orb(φ)∈CX/HG∑w(φ)=j=1∑kw(φj).
设 C C C为颜色集, X X X是染色对象集, w ( ⋅ ) w(\cdot) w(⋅)是定义在 C C C上的权函数,且 w ( ⋅ ) w(\cdot) w(⋅)在群 H H H的每条轨道上是常数。对于给定的 τ σ ∈ H G \tau^{\sigma}\in H^{G} τσ∈HG,令 F i x ( τ σ ) = { φ ∣ φ ∈ C X , τ σ ( φ ) = φ } \mathrm{Fix}(\tau^{\sigma})=\{\varphi|\varphi\in C^X,\ \tau^{\sigma}(\varphi)=\varphi\} Fix(τσ)={ φ∣φ∈CX, τσ(φ)=φ}, w ( F i x ( τ σ ) ) w(\mathrm{Fix(\tau^\sigma)}) w(Fix(τσ))表示 τ σ \tau^\sigma τσ的所有不动点的权和,则有
I n v ( C X / H G ) = 1 ∣ H G ∣ ∑ τ σ ∈ H G w ( F i x ( τ σ ) ) . \mathrm{Inv}(C^X/H^G)=\frac{1}{|H^{G}|}\sum\limits_{\tau^{\sigma}\in H^{G}}w\big(\mathrm{Fix}(\tau^{\sigma})\big). Inv(CX/HG)=∣HG∣1τσ∈HG∑w(Fix(τσ)).
对于 τ σ ∈ H G \tau^\sigma\in H^G τσ∈HG, F i x ( τ σ ) \mathrm{Fix}(\tau^\sigma) Fix(τσ)表示置换 τ σ \tau^\sigma τσ不动点的集合, w ( F i x ( τ σ ) ) w(\mathrm{Fix(\tau^\sigma)}) w(Fix(τσ))表示 τ σ \tau^\sigma τσ的所有不动点的权和,则有
w ( F i x ( τ σ ) ) = ∏ j = 1 n [ ∑ τ j ( n = b ) = b w ( b j ) ] λ j ( σ ) = ∏ j = 1 n M j ( τ ) λ j ( σ ) , w(\mathrm{Fix}(\tau^\sigma))=\prod_{j=1}^n\bigg[\sum_{\tau^j(n=b)=b}w(b^j)\bigg]^{\lambda_j(\sigma)}=\prod_{j=1}^nM_j(\tau)^{\lambda_j(\sigma)}, w(Fix(τσ))=j=1∏n[τj(n=b)=b∑w(bj)]λj(σ)=j=1∏nMj(τ)λj(σ),
其中
M j ( τ ) = ∑ τ j ( b ) = b w ( b ) j . M_j(\tau)=\sum_{\tau^j(b)=b}w(b)^j. Mj(τ)=τj(b)=b∑w(b)j.
M j ( τ ) M_j(\tau) Mj(τ)是用 τ \tau τ的所有 d d d循环中的颜色循环顺序地染色 σ \sigma σ的一个 j j j循环中的对象由此得到的所有染色方案的权和,其中 d ∣ j d\,|\,j d∣j.
由上述引理1,2可以得到下面的定理。
设 ( G , ∗ ) (G,\,\ast) (G,∗)和 ( H , ∙ ) (H,\,\bullet) (H,∙)分别是 n n n元对象集 X X X和 m m m元颜色集 C C C上的置换群, w ( ⋅ ) w(\cdot) w(⋅)是定义在 C C C上的权函数,且 w ( ⋅ ) w(\cdot) w(⋅)在群 H H H的每条轨道上是常数,则
I n v ( C X / H G ) = 1 ∣ H ∣ ∑ τ ∈ H C I G ( M 1 ( τ ) , M 2 ( τ ) , ⋯ , M n ( τ ) ) , \mathrm{Inv}\left(C^X/H^G\right)=\frac1{|H|}\sum_{\tau\in H}\mathrm{CI}_G(M_1(\tau),\,M_2(\tau),\,\cdots,\,M_n(\tau)), Inv(CX/HG)=∣H∣1τ∈H∑CIG(M1(τ),M2(τ),⋯,Mn(τ)),
其中 C I G ( x 1 , x 2 , ⋯ , x n ) \mathrm{CI}_G(x_1,\,x_2,\,\cdots,\,x_n) CIG(x1,x2,⋯,xn)是置换群 G G G的循环指数, M j ( τ ) = ∑ τ j ( b ) = b w ( b ) j M_j(\tau)=\sum\limits_{\tau^j(b)=b}w(b)^j Mj(τ)=τj(b)=b∑w(b)j.
即置换群 G G G只包含单位元 ι \iota ι, 此时有 C I G ( x 1 , x 2 , ⋯ , x n ) = x 1 n \mathrm{CI}_G(x_1,\,x_2,\,\cdots,\,x_n)=x_1^n CIG(x1,x2,⋯,xn)=x1n.
I n v ( C X / H G ) = I n v ( C X / H ) = 1 ∣ H ∣ ∑ τ ∈ H [ w ( F i x ( τ ) ) ] n . \mathrm{Inv}\left(C^X/H^G\right) =\mathrm{Inv}\left(C^X/H \right) =\frac1{|H|}\sum_{\tau\in H} \bigg[w\big(\mathrm{Fix}(\tau)\big)\bigg]^n. Inv(CX/HG)=Inv(CX/H)=∣H∣1τ∈H∑[w(Fix(τ))]n.
即置换群 H H H只包含单位元 ι \iota ι.
I n v ( C X / H G ) = I n v ( C X / G ) = C I G ( w 1 ( C ) , w 2 ( C ) , ⋯ , w n ( C ) ) , \mathrm{Inv}\left(C^X/H^G\right)=\mathrm{Inv}\left(C^X/G\right)=\mathrm{CI}_G\big(w_1(C),\,w_2(C),\,\cdots,\,w_n(C)\big), Inv(CX/HG)=Inv(CX/G)=CIG(w1(C),w2(C),⋯,wn(C)),
上式即为母函数型的Pólya定理(Pólya基本定理)。
即置换群 G , H G,\,H G,H均只包含单位元 ι \iota ι.
直接由De Bruijn定理,可以得到
I n v ( C X / H G ) = I n v ( C X ) = 1 ∣ H ∣ ∑ τ ∈ H [ w ( F i x ( τ ) ) ] n = [ ∑ c ∈ C w ( c ) ] n = ( x 1 + x 2 + ⋯ + x m ) n = ∑ n i ⩾ 0 , i = 1 , 2 , ⋯ , m ( n 1 + n 2 + ⋯ + n m n 1 , n 2 , ⋯ , n m ) x 1 n 1 x 2 n 2 ⋯ x m n m \begin{aligned} \mathrm{Inv}\left(C^X/H^G\right) &=\mathrm{Inv}\left(C^X\right) =\frac1{|H|}\sum_{\tau\in H} \left[w\big(\mathrm{Fix}(\tau)\big)\right]^n\\ &=\left[\sum_{c \in C}w(c)\right]^n =(x_1+x_2+\cdots+x_m)^n\\ &=\sum_{n_i\geqslant0,\,i=1,\,2,\,\cdots,\,m} \binom{n_1+n_2+\cdots+n_m}{n_1,\,n_2,\,\cdots,\,n_m}x_1^{n_1}x_2^{n_2}\cdots x_m^{n_m} \end{aligned} Inv(CX/HG)=Inv(CX)=∣H∣1τ∈H∑[w(Fix(τ))]n=[c∈C∑w(c)]n=(x1+x2+⋯+xm)n=ni⩾0,i=1,2,⋯,m∑(n1,n2,⋯,nmn1+n2+⋯+nm)x1n1x2n2⋯xmnm
此即在没有任何群的作用下 n n n元集到 m m m元集的所有映射方案的枚举,亦即 n n n个不同的球放入 m m m个不同的盒子且允许空盒的放入方案数的枚举。
将3个白球和1个黑球放入2个方形盒子和1个圆形盒子且允许空盒的模式清单(假定3个白球、2个方形盒子均不可区分)。
令 X = { b , w 1 , w 2 , w 3 } X=\{b,\,w_1,\,w_2,\,w_3\} X={ b,w1,w2,w3}为对象集, C = { r , s 1 , s 2 } C=\{r,\,s_1,\,s_2\} C={ r,s1,s2}为颜色集。显然 X X X与 C C C上的置换群分别为 G = S 1 ⊕ S 3 , H = S 1 ⊕ S 2 G=\mathcal{S_1}\oplus\mathcal{S_3},\ H=\mathcal{S_1}\oplus\mathcal{S_2} G=S1⊕S3, H=S1⊕S2.即
H = { ( r ) ( s 1 ) ( s 2 ) , ( r ) ( s 1 s 2 ) } ≜ { τ 1 , τ 2 } G = { ( b ) ( w 1 ) ( w 2 ) ( w 3 ) , ( b ) ( w 1 w 2 w 3 ) , ( b ) ( w 1 w 3 w 2 ) , ( b ) ( w 1 ) ( w 2 w 3 ) , ( b ) ( w 2 ) ( w 1 w 3 ) , ( b ) ( w 3 ) ( w 1 w 2 ) } \begin{aligned} H=\{&(r)(s_1)(s_2),\,(r)(s_1s_2)\}\triangleq\{\tau_1,\,\tau_2\}\\ G=\{&(b)(w_1)(w_2)(w_3),\,(b)(w_1w_2w_3),\,(b)(w_1w_3w_2),\\ &(b)(w_1)(w_2w_3),\,(b)(w_2)(w_1w_3),\,(b)(w_3)(w_1w_2)\} \end{aligned} H={ G={ (r)(s1)(s2),(r)(s1s2)}≜{ τ1,τ2}(b)(w1)(w2)(w3),(b)(w1w2w3),(b)(w1w3w2),(b)(w1)(w2w3),(b)(w2)(w1w3),(b)(w3)(w1w2)}
所以
C I G ( x 1 , x 2 , x 3 , x 4 ) = 1 6 ( x 1 4 + 2 x 1 x 3 + 3 x 1 2 x 2 ) , \mathrm{CI}_G(x_1,\,x_2,\,x_3,\,x_4)=\frac16(x_1^4+2x_1x_3+3x_1^2x_2), CIG(x1,x2,x3,x4)=61(x14+2x1x3+3x12x2),
定义 C C C上的权函数为
w ( r ) = x , w ( s 1 ) = w ( s 2 ) = y , w(r)=x,\ w(s_1)=w(s_2)=y, w(r)=x, w(s1)=w(s2)=y,
显然满足权函数在 H H H的每一条轨道上是常数。
由定义 M j ( τ ) = ∑ τ j ( b ) = b w ( b ) j M_j(\tau)=\sum\limits_{\tau^j(b)=b}w(b)^j Mj(τ)=τj(b)=b∑w(b)j, 得到
{ M 1 ( τ 1 ) = x + 2 y , M 2 ( τ 1 ) = x 2 + 2 y 2 , M 3 ( τ 1 ) = x 3 + 2 y 3 , M 4 ( τ 1 ) = x 4 + 2 y 4 , { M 1 ( τ 2 ) = x , M 2 ( τ 2 ) = x 2 + 2 y 2 , M 3 ( τ 2 ) = x 3 , M 4 ( τ 2 ) = x 4 + 2 y 4 . \begin{cases} M_1(\tau_1)=x+2y,\\ M_2(\tau_1)=x^2+2y^2,\\ M_3(\tau_1)=x^3+2y^3,\\ M_4(\tau_1)=x^4+2y^4,\\ \end{cases}\qquad \begin{cases} M_1(\tau_2)=x,\\ M_2(\tau_2)=x^2+2y^2,\\ M_3(\tau_2)=x^3,\\ M_4(\tau_2)=x^4+2y^4. \end{cases} ⎩⎪⎪⎪⎨⎪⎪⎪⎧M1(τ1)=x+2y,M2(τ1)=x2+2y2,M3(τ1)=x3+2y3,M4(τ1)=x4+2y4,⎩⎪⎪⎪⎨⎪⎪⎪⎧M1(τ2)=x,M2(τ2)=x2+2y2,M3(τ2)=x3,M4(τ2)=x4+2y4.
由此得到
C I G ( M 1 ( τ 1 ) , M 2 ( τ 1 ) , M 3 ( τ 1 ) , M 4 ( τ 1 ) ) = 1 6 [ ( x + 2 y ) 4 + 2 ( x + 2 y ) ( x 3 + 2 y 3 ) + 3 ( x + 2 y ) 2 ( x 2 + 2 y 2 ) ] = x 4 + 4 x 3 y + 7 x 2 y 2 + 10 x y 3 + 8 y 4 , C I G ( M 1 ( τ 2 ) , M 2 ( τ 2 ) , M 3 ( τ 2 ) , M 4 ( τ 2 ) ) = 1 6 [ x 4 + 2 x 4 + 3 x 2 ( x 2 + 2 y 2 ) ] = x 4 + x 2 y 2 , \begin{aligned} &\mathrm{CI}_G(M_1(\tau_1),\,M_2(\tau_1),\,M_3(\tau_1),\,M_4(\tau_1))\\ =&\frac16\left[(x+2y)^4+2(x+2y)(x^3+2y^3)+3(x+2y)^2\big(x^2+2y^2\big)\right]\\ =&x^4 + 4 x^3 y + 7 x^2 y^2 + 10 x y^3 + 8 y^4, \\ \\ \\ &\mathrm{CI}_G(M_1(\tau_2),\,M_2(\tau_2),\,M_3(\tau_2),\,M_4(\tau_2))\\ =&\frac16\left[x^4+2x^4+3x^2\big(x^2+2y^2\big)\right]\\ =&x^4+x^2y^2, \end{aligned} ====CIG(M1(τ1),M2(τ1),M3(τ1),M4(τ1))61[(x+2y)4+2(x+2y)(x3+2y3)+3(x+2y)2(x2+2y2)]x4+4x3y+7x2y2+10xy3+8y4,CIG(M1(τ2),M2(τ2),M3(τ2),M4(τ2))61[x4+2x4+3x2(x2+2y2)]x4+x2y2,
于是根据母函数型的De Bruijn定理,有
I n v ( C X / H G ) = 1 ∣ H ∣ ∑ τ ∈ H C I G ( M 1 ( τ ) , M 2 ( τ ) , M 3 ( τ ) , M 4 ( τ ) ) = 1 2 ( x 4 + 4 x 3 y + 7 x 2 y 2 + 10 x y 3 + 8 y 4 + x 4 + x 2 y 2 ) = x 4 + 2 x 3 y + 4 x 2 y 2 + 5 x y 3 + 4 y 4 . \begin{aligned} \mathrm{Inv}\left(C^X/H^G\right) &=\frac1{|H|}\sum_{\tau\in H}\mathrm{CI}_G(M_1(\tau),\,M_2(\tau),\,M_3(\tau),\,M_4(\tau))\\ &=\frac12\left(x^4 + 4 x^3 y + 7 x^2 y^2 + 10 x y^3 + 8 y^4+x^4+x^2y^2\right)\\ &=x^4 + 2 x^3 y + 4 x^2 y^2 + 5 x y^3 + 4 y^4. \end{aligned} Inv(CX/HG)=∣H∣1τ∈H∑CIG(M1(τ),M2(τ),M3(τ),M4(τ))=21(x4+4x3y+7x2y2+10xy3+8y4+x4+x2y2)=x4+2x3y+4x2y2+5xy3+4y4.
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