I'm reading Shannon's article A Mathematical Theory of Communication, and I'm stuck at the telegraphy case example, on page 4.
Shannon writes a formula involving $N(t)$, the number of sequences of duration $t$. He says that
$ N(t)=N(t-2)+N(t-4)+N(t-5)+N(t-7)+N(t-8)+N(t-10). $
My question is: where does this formula arise from? I can't figure out the recursive relation.
(There are 4 possible signals for the channel: the dot, which uses 2 units of time; the dash, which uses 4 units of time; the letter space, 3 units of time; the word space, 6 units of time. No spaces follow each other)