Perrin Chalkboard


Welcome to the Perrin Chalkboard!  I will be presenting in this blog a series of chalkboards which discuss interesting properties of the Perrin sequence and related integer sequences. This blog starts as a simple discussion of the Perrin sequence (the original mention by Lucas in 1876 and Perrin in 1899). It is found that an immense amount of research on the associated elliptic curves has occurred over the last 115 years. The Perrin sequence ties together much of the mathematics discussed today as algebraic number theory and modular functions. It is also integral to the discussion of Fermat’s Last Theorem conjectured in 1637 but proved by Wiles in 1994.

Theorems will be presented without proofs. I think the subject matter will appeal to those interested in the properties of integer sequences, elliptic equations, and graph theory. Many sequences from OEIS (On-line Encyclopedia of Integer Sequences) will be discussed, uncovering hidden or less obvious properties.

The primary subject matter in this blog covers the properties of integer sequences. However, it is not until Chapter 17 that I cover the subject matter of the short paper published by Perrin in 1899.  This chapter then introduces the subject of integer partitions, followed by some geometric applications of the Perrin sequence and then turns to division algorithms derived from general properties of cubic equations and associated integer sequences.

I encourage any comments or suggestions to the chalkboard subjects.

Although the pdf files are freely provided, if you are interested or have questions regarding any chapter please feel free to contact me.

Richard Turk
March 2015

Updated July 2017

Click on the pdf to open the first chapter.. or  continue scrolling to find the latest Chapter and pdf!



Chapter 32- The q Octic Transform

Equating the octahedral form and the elliptic modular form of the j-invariant resulted in equations between different quadratic field q-octic continued fractions.  These q-octic forms are transformed through radical expressions defined by the R and C transform.  Interesting properties are shown for these transforms using integer and fractional arguments.   The transforms are modular-like in complex multiplication and can be applied to the proper calculation of the j-invariant for various class equations from the q-octic continued fraction.

(updated 6/15/2019)

The q-Octic Transforms

Elliptic Functions and the Ramanujan Octave


The complete elliptic integral of the first kind provides the k modulus which is shown to connect these integrals and functions to the modular functions previously used for calculating the class invariant and the octic q continued fraction.  The k modulus is an algebraic integer between 0 and 1.  Equations are developed for odd and even class invariants and their associated octahedron.  The connection to the Elliptic Theta functions and the j-invariant is also shown.

Elliptic Functions and the Ramanujan Octave

Appendix C29B- The Ramanujan Octave and Discriminants -4m

The calculation of the g invariant for discriminants -4m requires an adjustment to the Ramanujan octave invariant for odd discriminants.  The new invariant is shown to be useful for finding solvable polynomials if the q octic continued fraction is divided by harmonics of the 12-tone scale. The 1/12 and 1/8 powers of 12 are significant in finding monic polynomials of degree 4 or less.  The numbers 8 and 12 also appear in the elements of the octahedron.  This platonic solid has 8 faces, 12 edges and 6 vertices.  The solid may have a natural relation to the 12-tone scale only based on the number 12 but it also extends into the mathematics of modular functions.  The powers 1/12 and 1/24 are found in many modular equations such as the j-invariant where  is a factor used to impose independence of the invariant for elliptic curves in any coordinate system.  The powers 12, 24 and 8 appear in the Weber function relations to the j-invariant.  Although other variants of the Ramanujan octave have appeared in the literature, the results discussed in this paper do not have any significance beyond these mathematical observations but still serves as an intriguing connection of mathematics to architectural geometry and music harmony.

The Ramanujan Octave and Discriminants -4m

Appendix C29- The Ramanujan Octave

An analogy is made between the equations derived Chapter 29 with the musical equal tempered scale.  I call this the Ramanujan octave based on his octic q continued fraction and its relation to class invariants.  The existence theorem of solvability is demonstrated with two examples representing the product and quotient of the modulus of the octic continued fraction.  These examples show a universality to finding radical forms of the class invariants for any class number.  An infinite number of semitones are shown to create the Ramanujan octave!

The Ramanujan Octave and Examples of the Existence Theorem

Chapter 29 Expressing the Octic q Continued Fraction in Radical Form using a modified Ramanujan’s Class Invariant Method

Following the P-Q modular equations used by B. Berndt (Transactions of the American Mathematical Society, 349(6), June 1997) to determine 13 radical forms of class invariants  reported by Ramanujan, this Chapter discusses a new method of analysis.  By using the q octic continued fraction and the q cubic solution equation (qkQ) derived in Chapter 28, all 13 radical forms are solved using results from Berndt’s two theorems.   The radical forms for discriminants with prime divisors of 5 and 7 are presented without the need for modular P-Q equations.

A theorem of existence of a solution of the class invariant in radicals is presented based on an invariant of the class invariant and modulus of the octic q continued fraction.

expressing the octic q continued fraction in radical form using a q cubic solution equation

Appendix C28- Calculating a Class Invariant with Ramanujan’s Octic q Continued Fraction

This appendix illustrates a calculation of one of Weber’s class invariants and compares the result obtained by Weber with the analysis presented in Chapter 28.  The analytical method provides some extra parameters for finding a suitable irreducible polynomial that can be expressed in radical form.  Consequently, the modulus of the q continued fraction is also expressed in radicals and/or nested radicals.

calculating a class invariant from ramanujan’s octic q continued fraction