You've still got the plural to think about:

* á haustin, x 42 000

and don't neglect the accusative:

* í haust x 366 000

In my own idiolect, which I suppose is fairly typical:

í haust - this coming autumn
á haustin - during autumn (in general; every autumn)
um haustið - during the autumn (a particular autumn; last autumn)
að hausti - a slightly literary phrase; I don't think I'd use it in spoken
language

The other possibilities seem unusual or ungrammatical (um hausti? can 'um'
take dative?) though I suppose I might say 'um haust' with a similar
meaning to 'á haustin'. Sounds a bit stilted, though.

Also note that a nice idiomatic way to talk about autumn coming is by
using the verb 'hausta'.

Kveðja,
Haukur

> Aha, many autumns -- the autumnal motherlode!
>
> að hausti, x 50 000
> um haustið, x 26 000
> á hausti, x 741
> um haust, x 520
> að haustinu, x 383
> á haustinu, x 287
> í haustinu, x 113
> í hausti, x 106
> um hausti, x 39
> haustið í gegnum, x 1 of 17
> í gegnum haustið, x 1 of 8
>
> (No examples of those last two without the article.) Hmm, I think I
> just found a new and disturbing numerical-seasonal hobby. Any hints
> about different shades of meaning?
>
>
> --- In norse_course@yahoogroups.com, Haukur Þorgeirsson <haukurth@...>
> wrote:
>>
>> > I got 50 500 hits for 'að hausti', 741 'á hausti', 106 í
>> > 'hausti', and just 39 for 'um hausti'.
>>
>> Also try 'um haust' and 'um haustið'.
>>
>> Kveðja,
>> Haukur
>
>
>
>