This site is supported by donations to The OEIS Foundation.

# Doubly infinite sequences

From OeisWiki

**Doubly infinite sequences** (or **two-way infinite sequences**) have neither a first term, nor a last term, and are thus not wellordered. Doubly infinite sequences cannot be added directly^{[1]} to the OEIS, since it requires a first term.

Example:

The sequence of integers, i.e. elements of in ascending order, given by

- {..., -16, -15, -14, -13, -12, -11, -10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, ...}

## See also

## Notes

- ↑ A workaround is to use a pair (A??????, A??????) of backward and forward sequences.