[seqfan] Re: F Numbers

David Wilson dwilson at gambitcomm.com
Thu Jan 8 21:06:34 CET 2009

I really think the way this should be handled is to supply an index for
each sequence.

Sequences defined on the positive integers would have index sequence
A000027, those defined on the nonnegative integers A001477, etc.  This
would make it easy to define an integer function on, say, the primes or
the squarefree numbers, we could just give A000040 or A005117 as the
index sequence.

For multidimensional sequences, we might either specify tuple indexes by
functions on sequences such as

join(x, y, z, ...) = { (x(n), y(n), z(n), ...) }

or

tuple(x, k) = { (x(kn), x(kn+1), ..., x(kn+(k-1))) }

so that

join(A002262, A025881) = tuple(A057554, 2)  = { (0,0), (0,1), (1,0),
(0,2), (1,1), (2,0), ... }

is the standard diagonal index of N x N (here N includes 0). You could
define other index sequences to traverse a multidimensional domain in
any way that is amenable to the function at hand.

To index on the nonnegative rationals, we could use index join(A007305,
A007306).

Sequences like "a(n) = some function of prime(n)" could be changed to
"a(n) = some function of n" with index A000040. Sequences like "a(n) =
some function of n, or -1 if undefined" could be changed to "a(n) = some
function of n" with an index sequence containing only the values for
which a(n) is defined.

At the moment, I have not thought about functions of partitions and
such. One idea is indexing on A025847, equating A025847(n) to the
partition described by the exponents in its standard prime factorization
(e.g, partition (2,1,1,1) is encoded as 2^2*3*5*7 = 420 = A025847(29)).

This approach is quite flexible, and only requires adding a new sequence
line for the index sequence.

franktaw at netscape.net wrote:
> I would like to propose a major enhancement to the OEIS.
> This is intended to deal with discrete functions that do not
> fit neatly into the OEIS format.  This includes functions
> whose domain and/or range consists of rational numbers,
> Gaussian (complex) integers, sequences (including
> partitions), polynomials, and perhaps other similar objects.
> It would also include sequences whose domain is integers,
> but not all consecutive integers starting at a specified value.
>
> These functions would be entered in a fashion similar to
> the way OEIS entries are now entered, but they would
> not have any values or offsets entered.  Instead, they
> would have a definition line.  The values would be
> computed by the system (either as requested or in
> advance; this is an implementation issue).
>
> I propose that these function be given F numbers
> (starting with F000001) instead of the A numbers used
> for the main body of the OEIS.  When appropriate, the
> system would generate the equivalent of b-files for them
> (call these f-files), reading the source b-files/f-files as
> needed.
>
> Possible definition lines would include:
>
> A#B
>
> (Here and in the following, A and B are other sequence/
> function references.  Note that when referencing signed
> OEIS sequences, it is always the signed values that are
> referenced.)  In this case, B must be a sequence of
> non-negative integers from the OEIS; A must be a
> sequence, but its values might be either integers or
> other discrete values.  Typically, A will have the "tabf"
> keyword.  The result will be a sequence of finite
> sequences (enclosed, perhaps, in square brackets), each
> representing one row of the table; B gives the row
> lengths.  The resulting offset will be the offset of B.
>
> A/B
>
> A and B should here be integer-valued, usually with the
> "frac" keyword; the result is a sequence of rational
> numbers with sequence A as numerators and B as
> denominators.  A and B should have the same offset.
>
> A+Bi
>
> Similar to the previous case, although rational values
> would also be allowed.  The result is a sequence of
> complex numbers (always rational).
>
> A:B
>
> A and B should both be indexed by integers, with
> the same offset.  The result is a function, with the
> A values forming the domain, and the B values the
> range.  Entries of this type would not have values
> directly in the entry; instead, an f-file would always
> be generated, and the values placed there.
>
> Ax^k
>
> Here k is an integer, usually zero or one.  A must be
> a function whose values are sequences of numbers.
> The result is a sequence of polynomials, with k being
> the exponent corresponding to the first term in each
> sequence.  If A  is a sequence, the result will be a
> sequence with the same offset.
>
> A|B
>
> A and B must be sequences, normally with the same
> kind of values.  The result is a function with
> f(n) = A(n) and f(-n) = B(n).  The values are stored
> in the f-file in the order 0,1,-1,2,-2,3,-3,....  Note
> that it is quite legal for the offset of A to be 1 (or
> even larger; likewise for the offset of B to be
> greater than 1), in which case the function will be
> undefined at 0 (and perhaps some adjacent values).
>
> A_1,A_2,...,A_k
>
> Each value is a k-tuple (k>1), with the values taken
> from the inputs.  These must all have the same domain.
>
> -----
> Searching
>
> Searching the function database should be fully
> integrated with searching the OEIS.  Sequences
> with non-integer values should be searchable in the
> usual way, so that one might search for
> "2+i,3+4i" or "1/2,2/3,1/2,4/5,1/3".
>
> To  handle functions that are not sequential, an
> enhancement to the search syntax is proposed:
> allow searching for targets in the form key->value
> (or possibly key:value).  This is applicable to existing
> sequences, so that a search for "5->1024" would
> match all sequences with a(5)=1024.  Supporting
> this would require the search setup program to
> take into account the offset of the sequence, as
> well as to scan the b-file, when one exists.
>
> -----
> Transforms
>
> In order to avoid having to add trivial variants of
> existing sequences to the database, a few simple
> transforms should be specifiable when referencing
> sequences in the definition lines of functions.  I
> would propose specifying these with prefixes to
> the sequence.  In particular, I would support:
>
> D
>
> Drop a value from the front of the sequence, and
> increment the offset (so that the offsets of the
> remaining values stay the same.  For example,
> DA001477 would be synonymous with A000027.
>
> I(v)
>
> The inverse of the previous operation; add a new
> initial value and decrement the offset.  Thus
> I(0)A000027 is equivalent to A001477.
>
> This could perhaps be extended to allow
> I(x->y), to add one specific value to a function.
>
> L
>
> Left shift: decrement the offset.
>
> R
>
> Right shift: increment the offset.
>
> I don't think any but these completely trivial
> transforms should be supported.  Even a bisection
> ought to be done with a separate sequence.
>
> -----
> Issues
>
> I have debated whether a distinction should be
> made between functions whose codomain is
> finite sequences, and those where it is finite
> sets.  One problem with making such a distinction
> is that it is not always clear cut: should every
> function whose values are always strictly increasing
> (or decreasing) sequences be considered to be
> a function to sets?  How would you know which
> to search for?
>
> Sequence in context: I don't see any way to
> do this for these functions, with the sole
> exception of sequences of rationals.  I don't
> think it would hurt to simply exclude this
> functionality from these functions.  This implies
> that the offset for sequences in this portion of
> the database will be a single integer; and for
> more general functions, there will be no offset.
>
> For rational number sequences, must the source
> sequences be relatively prime?  Or do we let
> the system reduce them?  A related issue: is the
> fraction 1/0 to be allowed?  (There is at least
> one sequence in the database with the "frac"
> keyword, intended as denominators, which
> contains a zero.)
>
> More definition types can obviously be added,
> although this should be minimized to keep
> things simple for the implementation.  One
> possibility is A+Bsqrt(k), with k a constant.
>
> It would be useful to support graphing for
> functions that have both domain and codomain
> real numbers (integers or rationals).
>
> -----
> Examples
>
> A000040:A001918 Function from primes to their
> least primitive roots.
>
> A036036[A036043] List of partitions in
> Abramowitz and Stegun order.
>
> A076512/A109395 Fraction of numbers less than
> n that are relatively prime to n.
>
> A001477|A000027 The absolute value function
> on the integers.
>
> -----
> Errors
>
> There are a number of errors possible in entering
> these functions.  The following is not intended
> as a complete list:
>
> Invalid definition: the definition entered cannot
> be recognized as any of the legal types.
>
> Wrong type of sequence: the above definition
> types state certain constraints on the referenced
> sequences.  The input process must check these
> conditions and report an error if violated.
>
> Duplicate values: for the A:B definition type, the
> values of sequence A must all be distinct.  An
> error must be reported if they are not.  (Of
> course, only the entered values can be checked;
> there is no way to verify that duplication does not
> occur later in the sequence.)
>
> Circular reference: when an existing function is
> being changed, it is possible to set up a circular
> reference, where the sequence directly or
> indirectly references itself.  This must be
> prevented.
>
> -----
> Implementation
>
> If it is agreed that this is a useful direction for the
> OEIS, the next question becomes how to get it
> implemented.  I don't know what resources might
> be available for this, nor when they might be
> available.
>
> I am sure that if never proposed, it will never be
> implemented.
>
> Franklin T. Adams-Watters
>
>
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