Pentadecathlon (or PD; plural pentadecathlons[note 1]) is a period-15oscillator that was found in 1970 by John Conway[2] while tracking the history of short rows of cells (see one-cell-thick pattern); indeed, an orthogonal row of 10 cells evolves into this object. It is the only known oscillator that is a polyomino in more than one phase (besides the blinker), and is the smallest oscillator with a period greater than its minimum population.
The pentadecathlon is so called because it has a period of 15 generations. This, being a factor of 30, means that it can be elegantly used in combination with period 30 devices (based on the queen bee shuttle). Firstly, it can reflect a glider 180° as in p60 glider shuttle, and a pair of perpendicular pentadecathlons can rotate a glider 90° or 180° as in 6 bits and 106P135.
A pentadecathlon can interact with another pentadecathlon to form a larger non-trivial oscillator, with bi-pentadecathlon 1 being a notable such example.
Hassling capabilities
The pentadecathlon is classified as a pulsating oscillator, since it undulates throughout its cycle. During this process, the pentadecathlon throws off multiple accessible sparks. More specifically, the oscillator produces horizontal T-nose sparks and horizontal V sparks in the form of phi sparks, as well as vertical domino sparks. Two copies of these domino sparks can be used to hassletoads in two distinct ways. The V sparks elegantly convert blocks into gliders, which forms the basis of the p30 xWSS-to-glider converter and aforementioned glider reflectors. This property is also exploited in numerous oscillators.
The pentadecathlon can undergo phase-changing reactions. The first two in the gallery below are in the form 14+15N, while the last two are in the form 6+15N.
Pentadecathlon is the most common natural oscillator of period greater than 3 (and indeed, the second most common natural oscillator of period greater than 2) in Achim Flammenkamp's census. In fact, it is the fifth or sixth most common oscillator overall in this census, being about as frequent as the clock, but much less frequent than the blinker, toad, beacon or pulsar.[8] It is also the most common oscillator with a volatility of 1. The pentadecathlon is also the fifty-second most common object on Adam P. Goucher's Catagolue and by far the most common period 15 oscillator, with all other natural oscillators of that period featuring it combined with some other object.[9]
The frequency of the pentadecathlon significantly depends on soup parameters; they are more common in smaller soups and lower densities, while they are less common on a torus.[note 2]
A pulsar-on-pentadecathlon is the largest object to have occurred in the B3/S23/C1 census as of January 2022, with 100 cells in its maximum phase.[10][9]
A 5-glider synthesis was reported by Douglas G. Petrie in June 1973.[11] Soon Peter Raynham found a 4-glider recipe.[12]
On April 11, 1997, Heinrich Koenig found a three-glider collision that produced a clean pentadecathlon. This was a surprising result at the time. Four-glider pentadecathlon recipes had been known and used for many years, so this was an unlooked-for improvement, very similar to Luka Okanishi's discovery of a three-glider synthesis of a switch engine almost twenty years later. This also makes the pentadecathlon the rarest object in Catagolue for which a 3-glidersynthesis is known.
In other rules
In EightLife, it still functions as a period 15 oscillator, but it generates extra internal sparks.
