LABELED TREES AND RELATIONS ON GENERATING FUNCTIONS M.P. DELEST, J.M. FEDOU Bordeaux I University LABRI ÷ Laboratory of Computer Science Abstract. In this paper, we give a combinatorial interpretation for a property on generating functions gived by R. Stanley. Our proof is based upon the study of special kind of labeled trees and forests. R~sum6. Nous donnons ici une interpretation combinatoire d'une propri~t~ des fonctions g~n6ratrices circe par R. Stanley. Notre preuve utilise une classe particuliere d'arbres et de forSts ~tiquett#es. INTRODUCTION Trees are presents in many subjects of theorical computer science. There are the natural representation of a lot of objects such as programs, arithmetic expressions, words or algebraic expressions in language theory. The structure of tree is very efficient in the study of complexity of algorithm because they constitute a dynamic data structure which allows to organize informations and to measure precisely the complexity of algorithm (see for example [4][6][11]). Trees are also present in other fields as shown in the paper of Viennot [14]. In some purely combinatorics subjects, they are the nice objects for understanding formulas. For example labeled trees, as defined in [2], are usefull in many subjects, see for instance Cori and Vauquelin [3] for the construction of a bijection between planar graphs and well labeled trees, or Moon [9] for identities on labelled forests. Moreover, the bijections are the basis of new theories in combinatorics (see for example [7],[13]) and trees are very studied in this context. In this paper, we introduce a special kind of set of labeled trees, the k-shaped forests, in order to give a bijective proof and a generalization of a result about generating functions ÷ Unit6 de Recherche Associ6e au Centre National de la Recherche Scientifique n°~26. Post Mail : 351Cours de la Liberation, 33405 TALENCE Cedex, FRANCE. Electronic Mail : maylis~geocub.greco-prog.fr This work was supported by the "PRC de Math~matiques et Informatique'.
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Labeled Trees and Relations on Generating Functions
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LABELED TREES AND RELATIONS ON GENERATING FUNCTIONS
M . P . DELEST, J . M . FEDOU
B o r d e a u x I U n i v e r s i t y
LABRI ÷
L a b o r a t o r y o f C o m p u t e r S c i e n c e
A b s t r a c t . I n t h i s p a p e r , we g i v e a c o m b i n a t o r i a l
i n t e r p r e t a t i o n f o r a p r o p e r t y on g e n e r a t i n g f u n c t i o n s g i v e d
by R. S t a n l e y . Our p r o o f i s b a s e d u p o n t h e s t u d y o f s p e c i a l
k i n d o f l a b e l e d t r e e s a n d f o r e s t s .
R~sum6. Nous d o n n o n s i c i une i n t e r p r e t a t i o n c o m b i n a t o i r e
d ' u n e p r o p r i ~ t ~ d e s f o n c t i o n s g ~ n 6 r a t r i c e s c i r c e p a r R.
S t a n l e y . N o t r e p r e u v e u t i l i s e une c l a s s e p a r t i c u l i e r e
d ' a r b r e s e t de f o r S t s ~ t i q u e t t # e s .
INTRODUCTION
T r e e s a r e p r e s e n t s i n m a n y s u b j e c t s o f t h e o r i c a l c o m p u t e r
s c i e n c e . T h e r e a r e t h e n a t u r a l r e p r e s e n t a t i o n o f a l o t o f o b j e c t s
s u c h a s p r o g r a m s , a r i t h m e t i c e x p r e s s i o n s , w o r d s o r a l g e b r a i c
e x p r e s s i o n s i n l a n g u a g e t h e o r y .
T h e s t r u c t u r e o f t r e e i s v e r y e f f i c i e n t i n t h e s t u d y o f
c o m p l e x i t y o f a l g o r i t h m b e c a u s e t h e y c o n s t i t u t e a d y n a m i c d a t a
s t r u c t u r e w h i c h a l l o w s t o o r g a n i z e i n f o r m a t i o n s a n d t o m e a s u r e
p r e c i s e l y t h e c o m p l e x i t y o f a l g o r i t h m ( s e e f o r e x a m p l e
[ 4 ] [ 6 ] [ 1 1 ] ) . T r e e s a r e a l s o p r e s e n t i n o t h e r f i e l d s a s s h o w n i n
t h e p a p e r o f V i e n n o t [ 1 4 ] .
I n s o m e p u r e l y c o m b i n a t o r i c s s u b j e c t s , t h e y a r e t h e n i c e
o b j e c t s f o r u n d e r s t a n d i n g f o r m u l a s . F o r e x a m p l e l a b e l e d t r e e s , a s
d e f i n e d i n [ 2 ] , a r e u s e f u l l i n m a n y s u b j e c t s , s e e f o r i n s t a n c e
C o r i a n d V a u q u e l i n [ 3 ] f o r t h e c o n s t r u c t i o n o f a b i j e c t i o n
b e t w e e n p l a n a r g r a p h s a n d w e l l l a b e l e d t r e e s , o r Moon [ 9 ] f o r
i d e n t i t i e s o n l a b e l l e d f o r e s t s . M o r e o v e r , t h e b i j e c t i o n s a r e t h e
b a s i s o f n e w t h e o r i e s i n c o m b i n a t o r i c s ( s e e f o r e x a m p l e [ 7 ] , [ 1 3 ] ) a n d t r e e s a r e v e r y s t u d i e d i n t h i s c o n t e x t .
I n t h i s p a p e r , we i n t r o d u c e a s p e c i a l k i n d o f s e t o f
l a b e l e d t r e e s , t h e k - s h a p e d f o r e s t s , i n o r d e r t o g i v e a b i j e c t i v e
p r o o f a n d a g e n e r a l i z a t i o n o f a r e s u l t a b o u t g e n e r a t i n g f u n c t i o n s
÷ U n i t 6 d e R e c h e r c h e A s s o c i 6 e a u C e n t r e N a t i o n a l d e l a R e c h e r c h e
S c i e n t i f i q u e n ° ~ 2 6 .
P o s t M a i l : 3 5 1 C o u r s d e l a L i b e r a t i o n , 3 3 4 0 5 TALENCE C e d e x , FRANCE.
E l e c t r o n i c M a i l : m a y l i s ~ g e o c u b . g r e c o - p r o g . f r
T h i s w o r k w a s s u p p o r t e d b y t h e "PRC d e M a t h ~ m a t i q u e s e t I n f o r m a t i q u e ' .
194
g i v e n b y R. S t a n l e y i n [ 1 0 ] ( s e e p r o p o s i t i o n 19 a t t h e e n d o f
t h i s p a p e r ) . I n f a c t , we s t u d y t h e g e n e r a t i n g f u n c t i o n s
F~(X) = ~ Nk(n) X n, where ~ ) o
Nk(n ) = ~(f~+fn+...+fn )(fn+f.+...+fn )...(fn+fn+...+fn ). 1 2 k 2 3 k ÷ l S $ + | s + k - 1
R. S t a n l e y g i v e s t h e r e s u l t f o r k=2 a n d 3 , t h e p r o o f o n l y f o r
k = 2 , s a y i n g t h a t t h e f o r m u l a f o r k=3 a p p e a r s a f t e r a n e n o r m o u s
a m o u n t o f c a n c e l a t i o n a n d t e d i o u s c o m p u t a t i o n . M o r e o v e r h e s a y s
t h a t h e d o e s n o t k n o w n a s i m p l e r a l t e r n a t i v e m e t h o d . We g i v e i t
i n p a r a g r a p h 3 a n d i n s o m e w a y we g e n e r a l i z e h i s r e s u l t .
A f t e r s o m e d e f i n i t i o n s a n d n o t a t i o n s , i n s e c t i o n 2 a n d 3 ,
we b r i n g b a c k t h e p r o b l e m o f t h e d e t e r m i n a t i o n o f F ~ ( X ) t o t h e
e n u m e r a t i o n o f k - s h a p e d f o r e s t s a c c o r d i n g t o t h e n u m b e r o f s o n s
o f e a c h v e r t e x .
T h i s p r o b l e m o f e n u m e r a t i o n i s t h e n s o l v e d , i n s e c t i o n 4 ,
i n t h e p a r t i c u l a r c a s e s w h e r e k = 2 , 3 a n d 4 . We s o l v e a l i n e a r
s y s t e m o f e q u a t i o n s o b t a i n e d b y t h e m e a n o f t h e k - s h a p e d f o r e s t s .
T h e r e a d e r s a r e r e f e r e d t o [ 4 ] [ 5 ] [ 1 4 ] f o r e x a m p l e s o f s i m i l a r
m e t h o d s .
1. D E F I N I T I O N S AND NOTATIONS
A r o o t e d t r e e i s a c o n n e c t e d g r a p h G w i t h o u t c y c l e [ 1 ]
w i t h a d i s t i n g u i s h e d v e r t e x r c a l l e d r o o t . I f ( f , s ) b e l o n g s t o G,
f ( r e s p . s ) i s c a l l e d f a t h e r ( r e s p . s o n ) o f s ( r e s p . f ) . H e r e , we
w i l l c o n s i d e r t r e e s w h i c h h a v e s o m e t i m e s a l o o p o n t h e r o o t . A
s e t o f t r e e s i s c a l l e d a f o r e s t A / a b e / e d t r e e i s a t r e e w i t h
a n i n t e g e r , c a l l e d l a b e l , a s s o c i a t e d t o e a c h v e r t e x . The e n t e r i n g
d e g r e e d e g ( i ) o f a v e r t e x l a b e l l e d i i s t h e n u m b e r o f s o n s o f
t h i s v e r t e x . T h e d i f f e r e n c e d e g r e e ~ ( i ) o f a v e r t e x l a b e l e d i i s
t h e d i f f e r e n c e b e t w e e n t h e l a b e l o f t h e f a t h e r o f t h i s v e r t e x a n d
i . T h e d i f f e r e n c e d e g r e e o f a l o o p i s e q u a l t o 0 . I t i s e a s y t o
g e n e r a l i z e t h e s e d e f i n i t i o n s t o t h e f o r e s t s . We n o t e ~ t h e e m p t y
f o r e s t .
L e t K b e a n h a l f r i n g a n d Y b e { Y l , Y2 , ' ' ' , y k } . We
d e n o t e b y K[Y] t h e r i n g o f t h e f o r m a l p o w e r s e r i e s o v e r Y w i t h
c o e f f i c i e n t s i n K. L e t f ( x ) = ~ , ~ o f n x " a n d g ( x ) = ~ , ~ o g , x " b e two
f o r m a l p o w e r s e r i e s f r o m K [ { x } ] , t h e p r o d u c t o f H a d a m a r d o f f a n d
g i s d e f i n e d b y
f ~ g ( x ) = ~ o f n g . x " "
G e n e r a l y s p e a k i n g , we w i l l d e n o t e b y ( ~ f ) P t h e p r o d u c t o f
H a d a m a r d p t i m e s o f t h e s e r i e f t h a t i s ( ~ f ) P = ~ n ~ c f , pxR" By
c o n v e n t i o n , we n o t e
A = ( ~ f ) o = ~ > I x ~ = x / ( l - X ) .
We c a l l p a r a m e t e r a n a p p l i c a t i o n f r o m ~ i n t o ~ . T h e t w o d e g r e e s
d e g a n d 6 a r e p a r a m e t e r s .
195
Q 3 7 / /
1 6
/ \ 4 5
/ 2
F i g u r e 1 . A f o r e s t o f ~ 3 , 8 "
2 . K-SHAPED FORESTS
We i n t r o d u c e a s p e c i a l k i n d o f l a b e l e d f o r e s t , i n o r d e r
t o " e x p l a i n " p r o d u c t s s u c h a s ( y l + . . . + y k ) . . . ( y ~ + . . . ÷ y , , ~ _ l ) .
D e f i n i t i o n 1 . L e t ~ be a f o r e s t h a v i n g t v e r t i c e s [ a b e ~ e d f r o m 1
t o t . T h e f o r e s t ~ i s k - ~ a b e ~ e d w h e n , f o r e u e r y i i n [ 1 . . t ]
0 ~ ~ ( i ) < k .
D e f i n i t i o n 2 . A k - C a b e ~ e d f o r e s t ~ i s s a i d k - s h a p e d i f ~ h a s
s + k - 1 v e r t i c e s w i t h s~O s u c h t h a t
i ) e v e r y r o o t i n ~ ~ a b e L e d w i t h 1 i n [ 1 . . s ] h a s a C o o p ,
i i ) t h e k - 1 ~ a s t v e r t i c e s ~ a b e ~ e d f r o m s + l t o s + k - 1 a r e
r o o t s w i t h o u t C o o p ,
N o t a t i o n s .
~ . ~ i s t h e s e t o f k - s h a p e d f o r e s t s h a v i n g s + k - 1
v e r t i c e s .
~ k . 0 i s t h e u n i q u e f o r e s t m a d e w i t h k - 1 i s o l a t e d
v e r t i c e s , we d e n o t e i t b y e .
~K i s t h e s e t o f k - s h a p e d f o r e s t s .
See f o r e x a m p l e t h e f i g u r e 12 t h e f o r e s t ~ b e l o n g s t o ~ 3 , e *
R e m a r k S . L e t ~ b e i n ~ k , s , we o b t a i n k f o r e s t s i n ~ k , s , l b y t h e
m e a n o f t h e f o l l o w i n g a l g o r i t h m :
b e g i n
f o r e v e r y v e r t e x i n ~ do l a b e l ( v e r t e x ) : = l a b e l ( v e r t e x ) + 1 ;
L e t 1 be t h e new v e r t e x ; t h e n 1 i s
( i ) e i t h e r a l o o p ,
( L L ) e i t h e r a s o n o f o n e o f t h e v e r t i c e s n u m b e r e d 2 t o k . e n d
T h u s , t h e n u m b e r o f k - s h a p e d f o r e s t s h a v i n g n + k - 1 v e r t i c e s i s k " .
E l s e , t h e v e r t e x l a b e l e d 1 i s a s o n o f a v e r t e x i i n [ 2 . . k S and
H d e ~ ( ~ ) ( Y l , . . . , Y s ÷ k _ l ) = Y i H ~ e g ( ~ ' ) ( Y ~ , . . . , Y s , k _ l ) -
Thus t h e lemma 11 i s p r o v e d . We d e d u c e
c o n s t r u c t i o n o f ~ k , s J e a c h f o r e s t ~ o f
C o n s e q u e n t l y we h a v e
Fk, ~ (x)
El 1 deg(i) X i p
i
P i l l
= A f (~).
,~E~'~,
s+k-1
i = l
and t h u s by f a c t o r i s a t i o n , we o b t a i n
I s÷k-1
n~l p1+..+ps+~_1=n ~E~P~ . ~
200
Using the notation F~(x) = ~ Fk,s(x), we get s)O
F~(x)=~ Af(~)
~ E ~ k
a n d t h e o r e m 10 f o l l o w s f r o m p r o p e r t y 7 .
4. RELATIONS BETWEEN SUBSETS OF ~
I n t h i s s e c t i o n , we p a r t t h e
s u b s e t s w h i c h b a s i c p r o p e r t i e s a l l o w u s t o d e c o m p o s e t h e
p o w e r s e r i e @ ( ~ k ) a s s u m o f f o r m a l p o w e r s e r i e s P i i . . . i
a r e given b y 1 2
set ~k into particular
formal
whose
\ Pi i . . . i = ) ~(~)
1 £ L__
~EEi . . . i I p
T h e n , we p r o v i d e r e l a t i o n s b e t w e e n t h e s e f o r m a l p o w e r s e r i e s a n d
we o b t a i n t w o t y p e s o f r e l a t i o n s :
- lemmas 12 to 15 will allow us to express P ......... I p p+q with respect to Pi ...i J
p p+q
l e m m a 16 w i l l a l l o w s u s t o w r i t e P i . . . i a s P~ . . . )
i n o r d e r t o a p p l y l e m m a s 12 t o 1 5 . 1 P+q 1 p÷q
M o s t o f t h e s e l e m m a s a r e b a s e d u p o n t h e r e l a t i o n o f
e q u i v a l e n c e ( d e f i n i t i o n 8 ) o f p a r t i c u l a r t y p e s o f f o r e s t s a n d a r e
d e d u c e d b y t h e a p p l i c a t i o n o f p r o p o s i t i o n 9 .
N o t a t i o n s : F o r e v e r y p o s i t i v e i n t e g e r p , a n d f o r e v e r y p - u p l e
i n [ O . . k - 1 ] ~ , we d e n o t e b y
Ei "''i the set of forests belonging to ~k having at 1
least p+k-1 vertices such that V j E [1.,p], ~(j)=ij,
" ei ...i 1
t h e f o r e s t o f ~ x w i t h p + k - 1 v e r t i c e s s u c h t h a t
V j E [ 1 . . p ] , 6 ( j ) = i j .
By conventions let E (resp. e) be ~ (resp. ~,o)"
E x a m p l e : t h e f o r e s t ~ o f t h e e x a m p l e 1 i s e x a c t l y e 2 2 o 2 1 1 a n d
b e l o n g s t o t h e s e t s E j E 2 , E ~ 2 j E 2 2 0 , . . . .
201
Lemma 12. For euery p - u p ~ e
and
( i l , i ~ , . . , i p) of [ O . . k - l ] p,
k - 1
Ei 1 ...i p = ei i ...i p + ~ Ei i ...i p h • (I)
h=O
k-i
P t I . . . i p = ~o ( e i i . . . i P ) + ~ P i 1 . . . i p h - ( 2 )
h = O
i s I n d e e d ~ a f o r e s t o f E i . . . i 1 p
- e i t h e r t h e f o r e s t e~ . . . i :P
1 p
- o r a f o r e s t h a v i n g a t l e a s t p + k v e r t i c e s ~ s u c h t h a t t h e
h a l f d e g r e e o f t h e v e r t e x p + l i s i n [ O . . k - 1 ] .
k - i k-I
E x a m p l e s : E = e + E i , @ ( ~ ) = Yo + P i ,
i = O i = O
k - I k - I
E i = e i + E i j ' P i = Y l Yo ÷ P i j
j = O j = O
P
Lemma 1 3 . P p ( p - 1 ) . . . z o = Yo Y p , z C~(~k) ( 4 )
P r o o f : An e l e m e n t ~ o f E ~ ( ~ _ l ) . . . 1 o ( s e e f i g u r e 3) i s a f o r e s t
{ u j ~ ' } w h e r e u i s t h e l a b e l e d t r e e w h o s e r o o t l a b e l e d p + l h a s a
l o o p a n d p s o n s l a b e l e d f r o m 1 t o p a n d w h e r e ~" i s a n y f o r e s t o f
~k w h e r e e a c h l a b e l 1 o f a v e r t e x h a s b e e n c h a n g e d i n ( t + p )
T h u s we h a v e t h e a n n o u n c e d r e s u l t u s i n g t h e p r o p e r t y 7 .
p+l
1 p
U
F i g u r e 3 . An e l e m e n t o f E ~ ( p _ l ) . . . 1 o .
202
I n t h e s a m e w a y , we f i x j s u p p l e m e n t a r y v e r t i c e s a n d we g e t
Lemma 16. P~<p-~)...~o~ ...i = Y0PY~+I P i ...i j I j
E x a m p l e : P o = y l ¢ ( ~ k ) , P l o = y o y ~ ( ~ k ) , P 2 1 o = Y o 2 y s ~ ( ~ k ) ...
a n d a l s o P o l = y l P ~ , P l o 2 3 = y o y ~ P 2 3 . . .
(5 )
Lemma 1 5 . L e t i 1 = a + p - l , i ~ = a + p - 2 , . . . , i p = ~ a n d
i p + 1 ¢ ~ - l , i p + 2 ~ - 2 , . . . , i ~ ÷ ~ ¢ O ( 6 )
t h e n
P~ i . . . i = Y o P - t Y p P i . . . i ( 7 ) I 2 ~ + p p+l ~ + p
L e t ~ b e a f o r e s t f r o m Pi i . . . i s a t i s f y i n g ( 6 ) . I 2 ~ + p
T h e n t h e v e r t e x l a b e l e d p+~ h a s e x a c t l y p s o n s l a b e l e d f r o m 1 t o
p a s s h o w n i n f i g u r e 4 . I n d e e d , t h e c o n d i t i o n i p + q C a - q i n ( 6 )
m e a n s t h a t t h e v e r t e x l a b e l e d p + a h a s no s o n o t h e r t h a n t h o s e
l a b e l e d f r o m 1 t o p . T h u s ~ i s e q u i v a l e n t t o t h e f o r e s t o b t a i n e d
b y d i s p l a c i n g t h e v e r t i c e s l a b e l e d f r o m 1 t o p so t h a t t h e
v e r t e x p i s a l o o p h a v i n g p - 1 s o n s l a b e l e d f r o m 1 t o p - 1 a s s h o w n
i n f i g u r e 4 . T h e r e f o r e , we c a n u s e t h e l e m m a 14 w h i c h p r o v e s
l e m m a 15.
Example: Ps23s:yoy2P3~.
p+a
A.2 1 p
P i i . . . t i 2 ~ + p
0
1 p-1 / p+~
yo~ - lyp D ~i . . . i
p÷l ~+p
F i g u r e 4 . An e x a m p l e f o r l e m m a 1 6 .
2 0 3
Lemma 16. I f t h e f o r e s t ~ of ~ i s s u c h t h a t , f o r s o m m e p o s i t i v e
i n t e g e r s a , ~ , ¥ , ~ , f a t h e r ( a ) = ~ + B + ¥ + ~ a n d f a t h e r ( a + ~ ) = a + ~ + ¥ , t h e n ,
i s e q u i v a l e n t t o t h e f o r e s t o b t a i n e d b y t h e p e r m u t a t i o n of t h e
s u b t r e e s r a i s i n g f r o m t h e v e r t i c e s a a n d a + ~ .
The p r o o f i s i m m e d i a t e b e c a u s e t h i s t r a n s f o r m a t i o n d o e s n o t
a f f e c t t h e e n t e r i n g d e g r e e . Thus we h a v e t h e f o l l o w i n g
C o r o l l a r y 1 7 . I f i~=B+¥+~ a n d i~÷~=~ t h e n
e i . . l . . i . . i 1 • ~ ÷ ~
Ei .. | ..i .. i 1 • ~ + ~
P i . . i . . i . . i 1 • ~ ÷ ~
where i ' ~ = B + ¥ a n d i ' ~ j = ¥ + ~ .
= e i . . i . . i . . i • p 1 ~ a ÷ ~ p
J ~ i . . i . . i . . i •
p 1 ~ ~ + ~ p
= P i . . i ' . . i ' o . i • (a)
E x a m p l e : P 2 o = P l l
5 . ENUMERATION OF THE K-SHAPED FORESTS
In t h i s s e c t i o n , we e n u m e r a t e t h e k - s h a p e d f o r e s t s
a c c o r d i n g t o t h e d i s t r i b u t i o n o f e a c h v e r t i c e s i n t h e f o r e s t s .
The lemmas o f t h e s e c t i o n 4 a l l o w us t o w r i t e o u t a l i n e a r s y s t e m o f e q u a t i o n s s a t i s f i e d by ~ ( ~ k ) " H o w e v e r • t h e
n u m b e r o f e q u a t i o n s i n c r e a s e s e x p o n e n t i a l y w i t h k . Thus i t was i n e f f i c i e n t t o w r i t e i n Macsyma [ 1 2 ] an a l g o r i t h m o f g e n e r a t i o n
f o r s u c h a g e n e r a l s y s t e m . T h u s , we j u s t s o l v e e x p l i c i t l y t h e s y s t e m s f o r k = 2 , 3 and 4 .
T h e o r e m 18. T h e g e n e r a t i n g f u n c t i o n f o r k - s h a p e d f o r e s t s a r e
Y0 f o r k = 2 , ~ (Y~) = •
(I-Yl)2 - YoY2
yo s f o r k = 3 , ~b(Y3) =
(l_y i)s (1-yl-ylS)-2YoY s (l_y 12)_yos (Yss÷Y3)
yo 3 ( 1 + yl s - YoY2 )
a n d fo r . k = 4 • ~ ( ~ 4 > ---- •
Qo + Q1Yo + QsYo s + QsYo 3 + Q4Yo 4
w h e r e Qo= ( l - y l ) 2 ( 1 - y l - y 1 2 - y l S ) •
Qt = - 2 y o y ~ ( l - y l ) ( 2 + y t - y t ~ - 3 Y t 3 - 3 Y t 4 - 2 Y 1 S ) , Q~= 2 Y 3 ( Y I - 1 ) ( l + 3 Y I + Y I s + Y ~ s) - y s ~ ( 1 - 4 Y 1 s - 6 y 1 4 ) ,
Q3 = - ( Y ~ s + 4 y ~ Y 3 + Y 4 ) + 2YIY~(Y~-Y~ 2) -Y1 2 (Y4+4Y2Y3+4Y~3) ,
Q4 = (Y2+Y3)~ + Y~Y4 - 2Y3 ~.
204
We w r i t e s y s t e m s o f l i n e a r e q u a t i o n s b y u s i n g f i r s t t h e l e m m a 1 2 ,
t h e n b y a p p l y i n g l e m m a s 13 t o 1 6 .
F o r k = 2 ~ we h a v e
~ ( ~ 2 ) = Y0 + P0 + P 1 ,
P I = YoY1 + P10 + P l 1 ~
Po = YI~(~2)'
PIo = YoY2¢(~2)'
P11 = YIPI "
S o l v i n g i t , g i v e s t h e r e s u l t o f t h e o r e m 1 8 . We g i v e i n
t h e a n n e x t h e s y s t e m f o r k = 3 a n d 4 . T h e y h a v e b e e n s o l v e d u s i n g
t h e s y m b o l i c m a n i p u l a t i o n s y s t e m MACSYMA f r o m MIT [ 1 2 ] o We d e d u c e
f r o m t h e t h e o r e m 19 t h e e x p r e s s i o n o f F k ( X ) f o r k = 2 ~ 3 a n d 4
u s i n g t h e t h e o r e m 1 1 . N o t e t h a t i n t h e c a s e k = 3 , we f i n d t h e
r e s u l t o f R. S t a n l e y [ 1 0 ] t h a t i s
P r o p o s i t i o n 19 . D e f i n e N ( n ) = ~ ( f ~ + f ~ + f ~ ) ( f ~ + f . + f n ) ' ' ' ( f . + fR + f . ) ,
1 2 3 £ 3 4 s S + I S + £
w h e r e f i s a n y f u n c t £ o n f : ~ ~ ¢ a n d w h e r e t h e s u m i s o u e r u l $
o r d e r e d p a r t i t i o n s n l + n 2 + . . . + n s ÷ 2 = n o f n ( n i ~ l ) . By c o n v e n t i o n ,
a s u m m a n d w i t h s=O i s 1 . D e f i n e
i " F ( X ) = N ( n ) X ~ , f = 2 f ~ X ~ , A = X / ( 1 - X ) . n=l 4=I
L e t ~ d e n o t e H a d a m a r d p r o d u c t . T h e n
A 2
F (X) =
( l - f ) 2 ( 1 - f - f ~ ) - 2 A ( f z f ) ( 1 - f 2 ) - A S ( ( f z f ) ~ + ( f z f x f ) )
H o w e v e r , we d i d n o t f i n d a g e n e r a l f o r m u l a f o r F ~ ( X ) j
o r , a t l e a s t a r e c u r s i v e r e l a t i o n o n t h e s e f u n c t i o n s . Y e t ~ t h e
t i m e o f c o m p u t a t i o n w a s v e r y s l i g h t , a n d we t h i n k t h a t i t i s
p o s s i b l e t o c o m p u t e t h e r e s u l t f o r k = 5 .
REFERENCES [ 1 ] C. BERGE, G r a p h e s e t h y p e r g r a p h e s , D u n o d , P a r i s ( 1 9 7 0 ) .
[ 2 ] N . G . d e B R U I J N a n d B . J . M . MORSELT, A n o t e o n p l a n e t r e e s , J .
C o m b . T h . 2 ( 1 9 6 7 ) , 2 7 - 3 4 .
[ 3 ] R. CORI a n d B . VAUQUELIN, P l a n a r m a p s a r e w e l l l a b e l e d t r e e s ,
C a n a d i a n J . o f M a t h . 33 ( 1 9 8 1 ) 1 0 2 3 - 1 0 4 2 .
[ 4 ] M . P . DELEST , U t i l i s a t i o n d e s l a n g a g e s a l g ~ b r i q u e s e t d u o a l -
c u l f o r m e l p o u r l e c o d a g e e t l ' 6 n u m ~ r a t i o n d e s p o l y o m i n o s ,
T h ~ s e d ' E t a t j U n i v e r s i t ~ d e B o r d e a u x I , 1 9 8 7 .
[ 5 ] J . M . FEDOU, E n u m e r a t i o n d e o e r t a i n s p o l y o m i n o s s e l o n l e s
p a r a m ~ t r e s p ~ r i m ~ t r e e t a i r e , m 6 m o i r e d e D . E . A , B o r d e a u x I ,
1 9 8 7 .
205
[ 6 ] P . FLAJOLET, A n a l y s e d ' a l g o r i t h m e s de m a n i p u l a t i o n d ' a r b r e s
e t de f i c h i e r s , C a h i e r s du B . U . R . O . 3 4 - 3 5 ( 1 9 8 1 ) , 1 - 2 0 9 .
[ 7 ] A. JOYAL, Une t h ~ o r i e c o m b i n a t o i r e d e s s ~ r i e s f o r m e l l e s , A d v .
i n M a t h . , 42 ( 1 9 8 1 ) , 1 - 8 2 . [ 8 ] D. KNUTH, The a r t of c o m p u t e r p r o g r a m m i n g , V o l . 1, A d d i s o n
Wesley Reading, 1968.
[9] J.W. MOON, A note on an identity and labelled forests,
C a r i b b . J . M a t h . , 3, (2 ) 5 9 - 6 5 . [ 1 0 ] R . P . STANLEY, G e n e r a t i n g f u n c t i o n , S t u d i e s i n Comb, 1 0 0 - 1 4 8 ,
17 , M a t h . A s s o c . A m e r i c a W a s h i n g t o n DC 7 8 .
[ii] J.M. STEYEART, Complexit~ et structure des algorithmes,
Th~se Universit~ Paris VII, 1984.
[12] SYMBOLICS INC., Macsyma Reference manual version ten, third
printing, december 1984.
[13] G. VIENNOT, Heap of pieces: Basic definitions and combinato-
rial lemmas, in C o m b i n a t o i r e E n u m d r a t i v e , UQAM 1985,
Montreal, G. Labelle et P. Leroux ed., Lecture Notes in
M a t h e m a t i c s , n ° 1 2 3 4 , S p r i n g e r V e r l a g , 3 2 1 - 3 5 0 , 1 9 8 6 .
[ 1 4 ] G. VIENNOT, T r e e s , R i v e r s , RNAs and many o t h e r t h i n g s ,
p r e p r i n t , B o r d e a u x I , 1 9 8 7 .
ANNEXE
For k=3,
@(~a) = Yo ~+ Po + P1 + Ps ,
P1 = YoSYl + P l o + P i i + P 1 2 , P~ = Yo 'Yo + P~o + P s i + P 2 z ,
P$o = Yo2Yl 2 + P~oo + P ~ o i + P 2 o 2 , P21 = Yo3Y2 + P21o + P211 + P 2 1 2 ,
P~s = YoSYl ~ + P s s o + P 2 s i + P 2 2 ~ ,
Po = Yl ~(~a),
P1o = Yo Y2¢(~a),
P11 = Yo Pi,
Px~ = Yo P~,
P~oo = Pilo = Yl Pio,
Psol = P~ll = Y* Pil,
Psos = P~is = Yl P~s,
P~io = YoSY~ ~(~)'
Psii = YoYs Pi"
F o r k--4 ,
~(5'4) = YO a+ Po + Pi +P2 + Pa'
Po = Yl ~)<~4 >' P1 = YoaY l + P l o + P i i + P12 + P l a ,