00:01
Here we want to determine the number of partitions of a number n into parts of size at most two.
00:08
So if we look at one of these partitions, so each such partition looks like a bunch of twos and then a bunch of ones.
00:25
2, 2, 2, 2, and then 1.
00:32
Of course, it could be all 2s, if n is even, or it could be all ones in any case.
00:38
But what we see, and this is the key, is each partition is uniquely identified by the sum of the two's.
00:57
Identified by the, or i guess we'd say, yeah, i'll say the sum of the twos.
01:06
And so what do i mean by that? what i mean is let's look at the partition, i don't know, 2, 2, 1, if this is a partition of 5.
01:17
This is the only partition of five with parts of size at most two where the sum of the twos equals four.
01:26
So all of the partitions of five, which have parts of size at most two, would be 1 -1 -1 -1, where the sum of the twos is zero.
01:39
And we have 2 -1 -1, where the sum of the twos is 2, and finally we have 2 -2 -2 -2 -1.
01:48
Or the sum of the twos is four.
01:52
And so what this means is a -n.
01:58
So, a -n is equal to the number of even numbers less than n.
02:07
Or i should say less than or equal to n...