Fibonacci numbers run one, one, two, three, five, and onward, with each term the sum of the two before it.

The longer version

Start the list with one and one, then keep adding the previous two entries to produce the next. The rule needs no multiplication and no cleverness, yet it generates one, one, two, three, five, eight, thirteen, and onward without ever repeating a value. Changing the two starting values gives a different but related family, while the addition rule stays identical.

The sequence is named for a thirteenth century Italian writer who used it to model breeding rabbits, though Indian poets had described the same counts while arranging syllables centuries earlier. Biologists meet it in the spiral rows of seed heads and the scales of cones, where new growth settles into the gap left by older parts.

Where this entry sits

This section collects odd corners of mathematics, from stubborn prime numbers to counting systems older than modern schools. Expect famous constants, probability puzzles, shapes that break everyday intuition, and numbers far too large to picture. The full list sits on the Numbers and Math page, where every entry is listed in order.