Sequencer – purely algorithmic number sequence identification

Sequencer identifies number sequences. That is, given a list of numbers like

(a(n)) = 1, 2, 4, 8, 16, 32, ...

it finds a formula that generates them, in this case

a(n) = 2^(n-1)

Sequencer employs neither a library of sequences nor a limited set of algorithms to find a closed form. Instead, it generates all formulas up to a certain size and then checks them against the provided numbers.

For verification, the system uses a hybrid approach of a fast numerical checker followed by a symbolic verifier powered by the Symja computer algebra system. Coupled with some tricks and heuristics designed to quickly generate potentially interesting formulas, Sequencer can identify sequences with very complex closed forms in a matter of seconds when run on commodity hardware.

Sequencer is capable of finding closed forms that are beyond any existing system like OEIS, Superseeker and Wolfram Alpha. It is particularly strong where recurrence relations or unusual combinations of functions are involved. For example, none of the services mentioned above can currently make sense of the sequence

(a(n)) = 1, 1, 1, 3, 5, 15, 43, 273, ...

while Sequencer reveals that it satisfies the recurrence relation

a(1) = 1
a(2) = 1
a(3) = 1
a(n) = a(n-2)^2+a(n-1)+a(n-3)   for n >= 4

and provides the continuation

2137, 76709, 4643751, 5888916569, 21570312343279, ...

Symbolic input and output

Sequencer is not limited to processing integers but can identify sequences consisting of arbitrary Symja expressions (provided they can be evaluated numerically). For example, invoking the program with the arguments 0 1/2 sqrt(3)/2 1 produces

a(n) = Sin(1/6*Pi*(n-1))
Continuation: 1/2*3^(1/2), 1/2, 0, -1/2, (-1/2)*3^(1/2), ...

Note that parentheses in arguments need to be escaped (\() when running a program from a shell like bash.

Installation and usage

Sequencer requires Java to run. Download the latest standalone Sequencer JAR (sequencer.jar) from the releases page and execute it from a terminal with the numbers to be matched as arguments, i.e.

java -jar sequencer.jar 1 2 3 4 5

Running the program without arguments displays a help text explaining the various command line parameters that can be used to fine-tune how searches are performed.

API

Sequencer can also be used as a library, for which precompiled JARs (sequencer-library-X.X.X.jar) are available on the releases page.

The class Sequencer provides the method

def identifySequence(sequence: Seq[String]): Seq[SequenceIdentification]

that returns objects of type

case class SequenceIdentification(formula: String, continuation: Seq[String])

When instantiating, the class must be passed a Configuration object

case class Configuration(
	maximumComplexity: Int,
	maximumIdentifications: Int,
	predictionLength: Int,
	recurrenceDepth: Int,
	combinatorialFunctions: Boolean,
	numberTheoreticFunctions: Boolean,
	transcendentalFunctions: Boolean,
	parallelSearch: Boolean,
	numericalTest: Boolean,
	printProgress: Boolean,
	outputLaTeX: Boolean
)

that controls the behavior of identifySequence. For more details, see the source code.

Development

Sequencer is written in Scala. To compile Sequencer from source, you need Git, a JDK, the Scala compiler, and sbt. Once all of these are installed and on your PATH, you are ready to build and run Sequencer:

git clone https://github.com/p-e-w/sequencer.git
cd sequencer
sbt run

The standalone JAR can be created using

sbt assembly

The library JAR can be created using

sbt package

All generated JARs will be written to target/scala-X.XX/.

To develop Sequencer using a Scala IDE, have sbt generate project files with a plugin like sbteclipse or sbt-idea.

Credits

Besides its runtime and compilation environment (Java, Scala and sbt), Sequencer depends on the Symja computer algebra system and the scopt command line parser. The standalone release JARs are built using the excellent sbt-assembly plugin.

License

Copyright © 2015 Philipp Emanuel Weidmann ([email protected])

Released under the terms of the GNU General Public License, Version 3