\input zb-basic
\input zb-matheduc
\iteman{ZMATH 2015c.00841}
\itemau{Robin, Anthony}
\itemti{The neglected counting problem.}
\itemso{Math. Sch. (Leicester) 44, No. 1, 16-17 (2015).}
\itemab
From the text: Permutations and combinations are familiar terms to many who have studied the subject, and as a brief reminder the following types of problems are often studied: Using a typewriter with $n$ keys, how many different $r$ letter words can be made? We have $n$ different objects. How many ways are there of arranging $r$ of these on a mantelpiece? We have $n$ friends, but only space to invite $r$ to a party. How many ways are there of doing this? In summary, for any such problem we have to consider whether or not ``repeats" are allowed, and whether the ``order" of those picked matters. We could therefore draw up a small table to show these different types of problem. We immediately see that there is a gap in our table, namely for the case when order is unimportant, and repeats are allowed. That is what we consider in the rest of this article.
\itemrv{~}
\itemcc{K20}
\itemut{combinatorics; permutations; combinations; counting problems; combination coding}
\itemli{}
\end