# [seqfan] Re: Sequences that use '-1' to indicate 'impossible'.

Neil Sloane njasloane at gmail.com
Wed Sep 23 20:55:29 CEST 2015

```Charles, was this addressed to me?

To be clear: are you saying that all "... or -1 if no such number exists"
sequences should havw keyword:sign, even if no such n is known?

Me: no, certainly not. Only if there is a known negative term.
Actually, the original meaning of the keyword "sign"
was to indicate that there is a -1 in the DATA section.
If all the DATA terms are nonnegative, then the keyword
is nonn even if there is known to be a -1 later in the sequence.

I thought that was the definition of nonn and sign. When did it change?

Best regards
Neil

Neil J. A. Sloane, President, OEIS Foundation.
11 South Adelaide Avenue, Highland Park, NJ 08904, USA.
Also Visiting Scientist, Math. Dept., Rutgers University, Piscataway, NJ.
Email: njasloane at gmail.com

On Wed, Sep 23, 2015 at 2:47 PM, Charles Greathouse <
charles.greathouse at case.edu> wrote:

> To be clear: are you saying that all "... or -1 if no such number exists"
> sequences should havw keyword:sign, even if no such n is known? For
> example, would "a(n) = the smallest prime p such that the next prime after
> p is p + 2n, or -1 if no such number exists" have keyword:sign (and change
> to keyword:nonn once Polignac's conjecture is proved)?
>
> Charles Greathouse
> Analyst/Programmer
> Case Western Reserve University
>
> On Wed, Sep 23, 2015 at 11:35 AM, Neil Sloane <njasloane at gmail.com> wrote:
>
> > Many definitions say "a(n) = ... or -1 if no
> > such number exists"
> >
> > And -1 is a negative number, so the keyword "sign" is perfectly correct.
> >
> > This has been our convention for at least 30 years, so we do not want to
> > change it.
> >
> > Best regards
> > Neil
> >
> > Neil J. A. Sloane, President, OEIS Foundation.
> > 11 South Adelaide Avenue, Highland Park, NJ 08904, USA.
> > Also Visiting Scientist, Math. Dept., Rutgers University, Piscataway, NJ.
> > Email: njasloane at gmail.com
> >
> >
> > On Wed, Sep 23, 2015 at 10:04 AM, Charles Greathouse <
> > charles.greathouse at case.edu> wrote:
> >
> > > There are cases where it would be nice to have a sigil for "undefined",
> > > "does not exist", "positive infinity", etc. But I think there's too
> much
> > > code relying on the entries to be strictly numeric for this to happen.
> > >
> > > Charles Greathouse
> > > Analyst/Programmer
> > > Case Western Reserve University
> > >
> > > On Wed, Sep 23, 2015 at 8:12 AM, Sidney Cadot <sidney at jigsaw.nl>
> wrote:
> > >
> > > > Hi,
> > > >
> > > > Some sequences use '-1' to indicate that a certain entry does not
> > > > exist, e.g. A154852 and A237584.
> > > >
> > > > Should such sequences be considered signed (kw:sign) or unsigned
> > > (kw:nonn)?
> > > >
> > > > Wouldn't it be more appropriate to allow an explicit "null" value in
> > such
> > > > cases?
> > > >
> > > > Regards, SIdney
> > > >
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