Part 1:
A couple of weeks ago, my friend John (from UK) asked me if I could explain the Inclusion-Exclusion Principle to him. Wikipedia contains an article on the same topic but John felt that it wasn’t a very helpful introduction. So, as promised, here is the post on that topic, though I managed to finish it only after some delay. (Sorry, John!)
As the title of this post suggests, the inclusion-exclusion principle can simply be described as counting all the objects outside the oval regions! We will use Venn diagrams to explain what that means.
Note: denotes the greatest integer less than
.
Ok, let’s now “build” the principle step by step.
1. Suppose there are objects out of which there are exactly
objects of type
. How many objects are NOT of type
? The answer is obvious:
. The Venn diagram below depicts the answer pictorially. The rounded rectangular region (with the orange border) is the set of all
objects, and the oval region (with the blue border) is the set of all
objects of type
. Then, the remaining white region that is outside the oval region denotes the set of all objects that are NOT of type
, and clearly, there are
of ’em.

Indeed, let us take a simple example. Consider the set of first thirty natural numbers: . So,
. Now, out of these thirty integers, let
be the number of integers divisible by
. Then,
. It is easy to see that the number of integers NOT divisible by
equals
, which is what we would expect if we were to list all the integers not divisible by
. Indeed, those integers are
.
2. Now, suppose there are objects out of which there are exactly
objects of type
and
objects of type
. Also, suppose there are exactly
objects that are of both type
AND
. Then, how many objects are NOT of type
OR
? The Venn diagram below illustrates this case.

Again, we are counting the number of objects outside the two oval regions. To answer the above question, we first need to determine the number of objects inside the two oval regions, and then subtract this number from the total, which is . Now, one might be tempted to think that the number of objects inside the two oval regions is simply
. But this is only true if the two oval regions don’t intersect (i.e. they have no objects in common.) In the general case, however, the expression
counts
twice! And so, we must subtract
from the expression to get
as the exact number of objects inside the two oval regions. We can now see that the number of objects outside the two oval regions equals
, and we are done.
Continuing with our example used earlier, let be the number of integers divisible by
. Also, let
be the number of integers divisible by
AND
(i.e. we count multiples of
.) Now, note that
, and
.
Thus, using the formula derived above, the number of integers that are NOT divisible by OR
equals
. In fact, we can list these ten integers:
and
; and this confirms our answer.
3. Now, suppose there are objects out of which there are exactly
objects of type
,
objects of type
and
objects of type
. Also, let
denote the number of objects of type
AND
,
the number of objects of type
AND
,
the number of objects of type
AND
, and
the number of objects of type
AND
. Then, how many objects are NOT of type
OR
? This case is illustrated by the Venn diagram shown below.

Once again, let us ask, what is the number of objects inside the three oval regions. A possible answer is . Now this will only be true if the three oval regions are pairwise disjoint. In the general case, however, we will have to take care of overcounting, just as we did in
earlier. A brief thought will reveal that in the above expression, we have counted each of
and
twice and
thrice! To take care of this overcounting, we subtract each of
and
once from the expression, but in doing so, we also end up subtracting
thrice! We thus need to add
back into the expression to get
, and this expression yields the exact number of objects inside the three oval regions. Therefore, the number of objects outside the three oval regions equals
. And, we are done.
Again, continuing with our earlier example, let denote the number of integers divisible by
. Then,
. Also, let
denote the number of integers divisible by
AND
(i.e. we are counting multiples of
); then,
. Again, let
denote the number of integers divisible by
and
; then
. And, finally, let
denote the number of integers divisible by
and
; then
.
So, the number of integers NOT divisible by OR
equals
. Indeed, those eight integers are
and
.
It isn’t very hard to deduce the formula for the general case when we have a set of objects, out of which there are
objects of type
,
objects of type
, and so on. The proof of the general formula will follow in the next post, which may include a couple of problems/solutions involving this principle.
2 comments
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July 24, 2008 at 3:34 pm
The Inclusion-Exclusion Principle « The Unapologetic Mathematician
[…] determining the cardinality of unions in terms of the sets in question and their intersection: the Inclusion-Exclusion Principle. Predictably enough, this formula is reflected in the subspaces of a vector […]
February 10, 2014 at 5:04 am
nick
I wonder to what operation this corresponds to in the category of Set