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:

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

### File and data format

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

$r$`}mod{`

$m$}.{$k$}`.gz`

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

).
Files range in size from 50 to 200 megabytes, and need to be uncompressed
with `gzip`

.
After uncompressing,
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 $4392$,