A Haskell interface to the Online Encyclopedia of Integer Sequences (OEIS), http://www.research.att.com/~njas/sequences/. Comments, suggestions, or bug reports should be sent to Brent Yorgey, byorgey at gmail dot com.
 getSequenceByID :: String > Maybe SequenceData
 lookupSequenceByID :: String > Maybe OEISSequence
 extendSequence :: SequenceData > SequenceData
 lookupSequence :: SequenceData > Maybe OEISSequence
 getSequenceByID_IO :: String > IO (Maybe SequenceData)
 lookupSequenceByID_IO :: String > IO (Maybe OEISSequence)
 extendSequence_IO :: [Integer] > IO [Integer]
 lookupSequence_IO :: SequenceData > IO (Maybe OEISSequence)
 searchSequence_IO :: String > IO (Maybe OEISSequence)
 lookupOEIS :: String > IO [String]
 type SequenceData = [Integer]
 data Language
 = Mathematica
  Maple
  Other
 data Keyword
 data OEISSequence = OEIS {
 catalogNums :: [String]
 sequenceData :: SequenceData
 signedData :: SequenceData
 description :: String
 references :: [String]
 links :: [String]
 formulas :: [String]
 xrefs :: [String]
 author :: String
 offset :: Int
 firstGT1 :: Int
 programs :: [(Language, String)]
 extensions :: [String]
 examples :: [String]
 keywords :: [Keyword]
 comments :: [String]
Example usage
Suppose we are interested in answering the question, "how many distinct binary trees are there with exactly 20 nodes?" Some naive code to answer this question might be as follows:
import Data.List (genericLength)  dataless binary trees. data BTree = Empty  Fork BTree BTree deriving Show  A list of all the binary trees with exactly n nodes. listTrees :: Int > [BTree] listTrees 0 = [Empty] listTrees n = [Fork left right  k < [0..n1], left < listTrees k, right < listTrees (n1k) ] countTrees :: Int > Integer countTrees = genericLength . listTrees
The problem, of course, is that countTrees
is horribly inefficient:
*Main> :set +s *Main> countTrees 5 42 (0.00 secs, 0 bytes) *Main> countTrees 10 16796 (0.47 secs, 27513240 bytes) *Main> countTrees 12 208012 (7.32 secs, 357487720 bytes) *Main> countTrees 13 *** Exception: stack overflow
There's really no way we can evaluate countTrees 20
. The solution? Cheat!
import Math.OEIS  countTrees works ok up to 10 nodes. smallTreeCounts = map countTrees [0..10]  now, extend the sequence via the OEIS! treeCounts = extendSequence smallTreeCounts
Now we can answer the question:
*Main> treeCounts !! 20 6564120420
Sweet. Of course, to have any sort of confidence in our answer, more research is required! Let's see what combinatorial goodness we have stumbled across.
*Main> description `fmap` lookupSequence smallTreeCounts Just "Catalan numbers: C(n) = binomial(2n,n)/(n+1) = (2n)!/(n!(n+1)!). Also called Segner numbers."
Catalan numbers, interesting. And a nice formula we could use to code up a real solution! Hmm, where can we read more about these socalled 'Catalan numbers'?
*Main> (head . references) `fmap` lookupSequence smallTreeCounts Just ["A. Bernini, F. Disanto, R. Pinzani and S. Rinaldi, Permutations defining convex permutominoes, preprint, 2007."] *Main> (head . links) `fmap` lookupSequence smallTreeCounts Just ["N. J. A. Sloane, <a href=\"http://www.research.att.com/~njas/sequences/b000108.txt\">The first 200 Catalan numbers</a>"]
And so on. Reams of collected mathematical knowledge at your fingertips! You must promise only to use this power for Good.
Lookup functions
getSequenceByID :: String > Maybe SequenceDataSource
Look up a sequence in the OEIS by its catalog number. Generally this would be its Anumber, but Mnumbers (from the /Encyclopedia of Integer Sequences) and Nnumbers (from the Handbook of Integer Sequences/) can be used as well.
Note that the result is not in the IO
monad, even though the
implementation requires looking up information via the
Internet. There are no side effects to speak of, and from a
practical point of view the function is referentially transparent
(OEIS Anumbers could change in theory, but it's extremely
unlikely). If you're a nitpicky purist, feel free to use the
provided getSequenceByID_IO
instead.
Examples:
Prelude Math.OEIS> getSequenceByID "A000040"  the prime numbers Just [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47... Prelude Math.OEIS> getSequenceByID "A1"  no such sequence! Nothing
lookupSequenceByID :: String > Maybe OEISSequenceSource
Look up a sequence by ID number, returning a data structure containing the entirety of the information the OEIS has on the sequence.
The standard disclaimer about not being in the IO
monad applies.
Examples:
Prelude Math.OEIS> description `fmap` lookupSequenceByID "A000040" Just "The prime numbers." Prelude Math.OEIS> keywords `fmap` lookupSequenceByID "A000105" Just [Nonn,Hard,Nice,Core]
extendSequence :: SequenceData > SequenceDataSource
Extend a sequence by using it as a lookup to the OEIS, taking the first sequence returned as a result, and using it to augment the original sequence.
Note that xs
is guaranteed to be a prefix of extendSequence xs
.
If the matched OEIS sequence contains any elements prior to those
matching xs
, they will be dropped. In addition, if no matching
sequences are found, xs
will be returned unchanged.
The result is not in the IO
monad even though the implementation
requires looking up information via the Internet. There are no
side effects, and practically speaking this function is
referentially transparent (technically, results may change from
time to time when the OEIS database is updated; this is slightly
more likely than the results of getSequenceByID
changing, but still
unlikely enough to be essentially a nonissue. Again, purists may
use extendSequence_IO
).
Examples:
Prelude Math.OEIS> extendSequence [5,7,11,13,17] [5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71...
Prelude Math.OEIS> extendSequence [2,4,8,16,32] [2,4,8,16,32,64,128,256,512,1024,2048,4096,8192...
Prelude Math.OEIS> extendSequence [9,8,7,41,562]  nothing matches [9,8,7,41,562]
lookupSequence :: SequenceData > Maybe OEISSequenceSource
Find a matching sequence in the OEIS database, returning a data structure containing the entirety of the information the OEIS has on the sequence.
The standard disclaimer about not being in the IO
monad applies.
getSequenceByID_IO :: String > IO (Maybe SequenceData)Source
The same as getSequenceByID
, but with a result in the IO
monad.
lookupSequenceByID_IO :: String > IO (Maybe OEISSequence)Source
The same as lookupSequenceByID
, but in the IO
monad.
extendSequence_IO :: [Integer] > IO [Integer]Source
The same as extendSequence
, but in the IO
monad.
lookupSequence_IO :: SequenceData > IO (Maybe OEISSequence)Source
The same as lookupSequence
, but in the IO
monad.
searchSequence_IO :: String > IO (Maybe OEISSequence)Source
Look up a sequence in the OEIS using its search function
lookupOEIS :: String > IO [String]Source
Interpret a string as a OEIS request, and return the results as Strings
Data structures
type SequenceData = [Integer]Source
Programming language that some code to generate the sequence is written in. The only languages indicated natively by the OEIS database are Mathematica and Maple; any other languages will be listed (usually in parentheses) at the beginning of the actual code snippet.
OEIS keywords. For more information on the meaning of each keyword, see http://www.research.att.com/~njas/sequences/eishelp2.html#RK.
data OEISSequence Source
Data structure for storing an OEIS entry. For more information on the various components, see http://www.research.att.com/~njas/sequences/eishelp2.html.
OEIS  
