The conumerator and the codenominator

A. Muhammed Uludağ*, Buket Eren Gökmen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


In this paper, we answer the question: “what is the qth Fibonacci number, where q is a positive rational?”. The answer is the codenominator function, which is an integral-valued map. It is defined via a system of functional equations, analogous to a system satisfied by the numerator function. Many Fibonacci identities in the literature can be generalized to the codenominator. It is related to Bird's tree and the involution Jimm of the reals induced by the outer automorphism of PGL(2,Z). The “corational” function Jimm satisfies a set of modular functional equations and maps quadratic irrationals to quadratic irrationals in a non-trivial manner. Although having jumps at rationals, it has a derivative which vanishes almost everywhere. We give a formula which gives the amount of jump of Jimm in terms of the codiscriminant function, which is defined in terms of the codenominator.

Original languageEnglish
Article number103192
JournalBulletin des Sciences Mathématiques
Publication statusPublished - Nov 2022


  • Bird tree
  • Continued fractions
  • Dyer's automorphism
  • Fibonacci numbers
  • Golden ratio
  • Hinze's sequence

ASJC Scopus subject areas

  • Mathematics(all)


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