The article presents a theorem which concerns functions and sequences. This theorem can be used in order to create equalities with a rational numerical expression or a natural number on the one side and a nested radical on the other.
Functions are introduced to pupils at the secondary school only. However, it is possible to include the propedeutics of functions already at nursery or primary school. At nursery, children can work with dependences, i.e. create and complete numbers sequences by means of manipulation, movement or graphic records. At primary school, learners complete numbers sequences or tables and familiarise themselves with the linear function by solving simple tasks from real life.