olymmath

Putnam 1993 A6

  • Session A

The infinite sequence of 2's and 3's

2,3,3,2,3,3,3,2,3,3,3,2,3,3,2,3,3,3,2,3,3,3,2,3,3,3,2,3,3,2,3,3,3,2,…\begin{aligned}&2,3,3,2,3,3,3,2,3,3,3,2,3,3,2,3,3, \\ &3,2,3,3,3,2,3,3,3,2,3,3,2,3,3,3,2,\dots\end{aligned}

has the property that, if one forms a second sequence that records the number of 3's between successive 2's, the result is identical to the given sequence. Show that there exists a real number rr such that, for any nn, the nnth term of the sequence is 2 if and only if n=1+⌊rm⌋n = 1 + \lfloor rm \rfloor for some nonnegative integer mm. (Note: ⌊x⌋\lfloor x \rfloor denotes the largest integer less than or equal to xx.)

Curated text Kedlaya Putnam archive. Quoted with source.

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