Stars and bars method for Combinatorics problems
The problems and the method of their solutions in this lesson are of highest peaks in Combinatorics.
They are at the level of a School Math circle at the local or (better to say) of a renowned University.
Problem 1
How many solutions (using only positive integers) are there to the following equation
x1 + x2 + x3 + x4 + x5 = 36 ?
Solution
x1 + x2 + x3 + x4 + x5 = 36 (1)
Imagine 36 marbles on the table, placed in one straight line with small gaps between them.
Imagine you have 6 numbered bars, of which you place the first bar (bar N1) before the first marble and the last bar (bar N6)
after the last marble in the row.
Let
,
,
,
,
be some solution to the given equation.
You then place bar N2 after
-th marble in the gap in the row of marbles;
then you count next
marbles in the row of marbles after bar N2 and place bar N3 in the gap there;
then you count next
marbles in the row of marbles after bar N3 and place bar N4 in the gap there;
finally, you count next
marbles in the row of marbles after bar N4 and place bar N5 in the gap there.
At this moment, all 36 marbles are divided in 5 groups between bars (1-2), (2-3), (3-4), (4,5) and (5,6).
So, having the solution to equation (1) in positive integer number, you place 4 bars N2, N3, N4 and N5 in their
corresponding positions in gaps in the row of marbles.
Vise versa, if you place 4 bars B2, B3, B4 and B5 in gaps in the row of 36 marbles, you divide marbles in 5 groups,
and the numbers of marbles in each group form the solution to equation (1).
Thus, there is one-to-one correspondence between the set of solutions to equation (1) in positive integer numbers,
from one side, and all different possible placings of 4 bars in gaps in the row of 36 marbles.
Also note that no one pair of bars goes to any single gap between marbles,
because the numbers
,
,
,
,
all are positive (!)
Thus, the number of solutions to equation (1) is equal to the number of COMBINATIONS of 35 (= 36-1) gaps taken 4 at a time.
Hence, the number of all possible solutions to equation (1) in positive integer numbers is equal to
=
= 52360.
ANSWER. The number of different solutions to equation (1) is
= 52360.
Solved.
------------------
More general problem to find the number of positive integer solution to equation
+
+
+ . . . +
= n
where k <= n, can be solved in the same way and has the answer
.
We have then (n-1) gaps between "n" "marbles" and (k-1) movable dividing bars.
The method I used in the solution is called the "method of bars and stars".
You can read about it in this Wikipedia article
https://en.wikipedia.org/wiki/Stars_and_bars_%28combinatorics%29
The second problem seems to be similar to the first one, but is different in some important aspects.
Problem 2
How many solutions (using non-negative integers) are there to the following equation
x1 + x2 + x3 + x4 + x5 = 36 ?
Solution
x1 + x2 + x3 + x4 + x5 = 36 (1)
Imagine 36 marbles on the table, placed in one straight line with small gaps between them.
Imagine you have 6 numbered bars, of which you place the first bar (bar N1) before the first marble and the last bar (bar N6)
after the last marble in the row.
Let
,
,
,
,
be some solution to the given equation.
You then place bar N2 after
-th marble in the gap in the row of marbles;
then you count next
marbles in the row of marbles after bar N2 and place bar N3 in the gap there;
then you count next
marbles in the row of marbles after bar N3 and place bar N4 in the gap there;
finally, you count next
marbles in the row of marbles after bar N4 and place bar N5 in the gap there.
At this moment, all 36 marbles are divided in 5 groups between bars (1-2), (2-3), (3-4), (4,5) and (5,6).
Notice that if some
is zero, then the corresponding bars go to the common respective gap.
In particular, if
= 0, then the bars N1 and N2 are placed in one common gap close to each other BEFORE the first marble (!)
So, having the solution to equation (1) in non-negative positive integer number, you place 4 bars N2, N3, N4 and N5 in
their corresponding positions in gaps in the row of marbles.
Vise versa, if you place 4 bars B2, B3, B4 and B5 in gaps in the row of 36 marbles, you divide marbles in 5 groups,
and the numbers of marbles in each group form the solution to equation (1).
Thus, there is one-to-one correspondence between the set of solutions to equation (1) in non-negative integer numbers,
from one side, and all different possible placings of 4 bars in 36 gaps in the row between 36 marbles, plus one gap before the 1-st marble.
Thus we have 36+4 = 40 entities, 36 gaps and 4 bars; 36 gaps are indistinguishable and 4 movable bars
are indistinguishable, too.
The number of all possible indistinguishable arrangements of 40 items of two types with 36 indistinguishable of one type
and 4 indistinguishable of the other type is
=
= 91390.
Hence, the number of all possible solutions to equation (1) in positive integer numbers is equal to
= 91390.
ANSWER. The number of different solutions to equation (1) is 91390.
Solved.
------------------
More general problem to find the number of non-negative integer solution to equation
+
+
+ . . . +
= n
where k <= n, can be solved in the same way and has the answer
.
We have then n gaps between "n" "marbles" (including one gap BEFORE the first marble) and (k-1) movable dividing bars.
The method I used in the solution is called the "method of bars and stars".
You can read about it in this Wikipedia article
https://en.wikipedia.org/wiki/Stars_and_bars_%28combinatorics%29
My other additional lessons on Miscellaneous word problems in this site are
- I do not have enough savings now
- In a jar, all but 6 are red marbles
- How many boys and how many girls are there in a family ?
- What is the last digit of the number a^n ?
- Find the last three digits of these numbers
- What are the last two digits of the number 3^123 + 7^123 + 9^123 ?
- Advanced logical problems
- Prove that if a, b, and c are the sides of a triangle, then so are sqrt(a), sqrt(b) and sqrt(c)
- Calculus optimization problems for shapes in 2D plane
- Calculus optimization problems for 3D shapes
- Solving some linear minimax problems in 3D space
- Solving one non-linear minimax problems in 3D space
- Solving linear minimax problem in three unknowns by the simplex method
- The "pigeonhole principle" problems
- In the worst case
- Page numbers on the left and right facing pages of an opened book
- Selected problems on counting elements in subsets of a given finite set
- How many integer numbers in the range 1-300 are divisible by at least one of the integers 4, 6 and 15 ?
- Nice problems to setup them using Venn diagram
- Wrapping a gift
- In preparation for Halloween
- Nice entertainment problems related to divisibility property
- Math Olympiad level problem on caves and bats
- Math Olympiad level problem on caught fishes
- Math Olympiad level problem on pigeonhole principle
- Math Olympiad level problem on placing books in bookcase
- OVERVIEW of additional lessons on Miscellaneous word problems
Use this file/link ALGEBRA-I - YOUR ONLINE TEXTBOOK to navigate over all topics and lessons of the online textbook ALGEBRA-I.
Use this file/link ALGEBRA-II - YOUR ONLINE TEXTBOOK to navigate over all topics and lessons of the online textbook ALGEBRA-II.