Data from extensive computations on class groups of quadratic imaginary fields is available below. It is organized by fundamental discriminant $-d$, and divided into four groups based on congruences:

- $d\equiv 3\pmod 8$
- $d\equiv 7\pmod 8$
- $d\equiv 4\pmod {16}$
- $d\equiv 8\pmod {16}$

`cl7mod8.12.gz`

, the final extension because it has been
compressed with `gzip`

. The compressed files range in size
from 50 to 200 megabytes.
### File and data format

The $k$th file of data for $d\equiv r\pmod{m}$ has filename of the form
`cl{`

$r$`}mod{`

$m$}.{$k$} after uncompressing
with `gzip`

(for example, file $12$ for $d\equiv7\pmod8$ is `cl7mod8.12`

).
In this file there is one line per discriminant, with discriminants in order of their absolute
value. The discriminants and associated class group data may be extracted as
follows, where for $i\ge1$ we define $-d_i$ to be the $i$th discriminant of the file:

- Initialise $-d_0=k\cdot2^{28}+r$.
- For $i\ge1$, let the data in line $i$ of the file be
$a$ $b$ $c_1\ c_2\ \ldots\ c_t$ -
Then
- $d_{i} = d_{i-1}+m\cdot a$,
- $h(-d_{i}) = b$,
- the invariant factors for the class group are $[c_1, c_2,\ldots, c_t]$.

For example, the first two lines of file `cl4mod16.1`

are

$0$ | $12160$ | $380\ 4\ 4\ 2$ |

$2$ | $4392$ | $2196\ 2$ |

- $-d_0=1\cdot2^{28}+4=268435460$, and then
- $-d_1=-d_0+16\cdot0=268435460$, with class number $12160$,
- $-d_2=-d_1+16\cdot2=268435492$, with class number $4392$,