This site is supported by donations to The OEIS Foundation.

# Doubly infinite sequences

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 ${\displaystyle \scriptstyle \mathbb {Z} \,}$ in ascending order, given by ${\displaystyle \scriptstyle a(0)\,=\,0,\,a(\pm n)\,=\,\pm n,\,n\,\geq \,1.\,}$

{..., -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, ...}