On the structure of the complement $\overline{\Mfib}$ of the set $\Mfib$ of fibbinary numbers in the set of positive natural numbers

Abstract

The set $\Mfib$ of fibbinary numbers is defined via a bijection between the set $\BB{N}$ of natural numbers and $\Mfib$. Since the elements of $\Mfib$ do not exhaust $\BB{N}$, the structure of the complement $\overline{\Mfib}$ of $\Mfib$ in $\BB{N}$ is of interest. An explicit expression $\overline{\Mfib}=\bigcup_{k \geq 1}^\infty \Phi_k$ is obtained in terms of certain well-defined sets $\Phi_k, \; k \geq 1$. The key to its proof lies in first considering the odd numbers involved in this statement: a general treatment, with full justification, of the binary representations of the odd numbers is developed, and exploited in showing the expression quoted for $\overline{\Mfib}$ to be correct. The main results of the article can also be viewed as providing partitions of the set $\BB{N}$ of natural numbers, and also of its subset of odd numbers, that follow from the introduction of the set $\Mfib$, and of its subset of odd integers.