Googology Wiki
Googology Wiki

5 (five) is a positive integer following 4 and preceding 6. Its ordinal form is written "fifth" or "5th".


Visualization of 5

  • 5 is an odd prime number. Since 10 is a multiple of 5, divisibility by 5 can be easily tested in the decimal system — multiples of 5 always end in 0 or 5. 5 is a Fermat prime: 221 + 1 = 5.
  • A 5-sided star is known as a pentagram.
  • It is thought that 5 is the only odd untouchable number.[1]
  • The base five numeral system is called quinary or pental.

In googology

Googological functions returning 5

As a cash denomination

Some currencies, such as the euro and the United States dollar, have banknotes with this number in the denomination.

Some currencies, such as the Czech koruna, the Israeli new shekel (shekel-wise and formerly Agorot-wise) and the Russian ruble, have coins with this number in the denomination.


  1. MathWorld, Untouchable Number