# [seqfan] Re: In search of a formula for a number triangle

Charles Greathouse charles.greathouse at case.edu
Tue Jan 8 01:01:04 CET 2013

```No. a(5) = 13 so a(79) cannot be 13.

Charles Greathouse
Analyst/Programmer
Case Western Reserve University

On Mon, Jan 7, 2013 at 6:36 PM, Harvey P. Dale <hpd at hpdale.org> wrote:

> Al:
>
>         Does this Mma program properly generate the terms of the sequence?
>
>         Flatten[Table[{n,Range[10n,10n+n-2]},{n,20}]]
>
>         Best,
>
>         Harvey
>
>
> -----Original Message-----
> From: SeqFan [mailto:seqfan-bounces at list.seqfan.eu] On Behalf Of Alonso
> Del Arte
> Sent: Monday, January 07, 2013 5:40 PM
> To: Sequence Fanatics Discussion list
> Subject: [seqfan] In search of a formula for a number triangle
>
> The triangle of A077341
>
> 1
>
> 2 20
>
> 3 30 31
>
> 4 40 41 42
>
> 5 50 51 52 53
>
> ...
>
>  19 190 191 ...
>
> 200 201 202 ...
>
>  21 210 211 ...
>
> ...
>
> and the related sequence A077342, initial rows of the triangle, has the
> question "Can anyone derive a formula for a(n)?" I can't, but I believe I
> may at least be able to figure out which a(n) = n. This is what I've come
> up with:
>
> a(n) = n when n > 10 * floor(n/10) + floor(n/10) - 2.
>
> But of course this only takes into account the nearest previous row that
> could possibly contain n. Is that enough or does this formula need to be
> amended to take into account even earlier rows?
>
> Al
>
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```