75 (number)

75 (seventy-five) is the natural number following 74 and preceding 76.

In mathematics

75 is the fourth ordered Bell number, and counts the number of weak orderings on a set of four items.

As the sum of the first five pentagonal numbers, 75 is a pentagonal pyramidal number. It is also an enneagonal number.

It is also a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 7, 5, 12, 17, 29, 46, 75... But there is no integer that added up to its own digits adds up to 75, hence 75 is a self number.

Excluding the infinite sets, there are 75 uniform polyhedra (or 76 if edges are allowed to coincide).

In other fields

Seventy-five is also:

• The atomic number of rhenium
• The age limit for Canadian senators^[1]
• A common name for the Canon de 75 modèle 1897, a French World War 1 gun
• The departement number of the city of Paris

References

1. ↑ “Who are our senators and what do they do?”, Parliament of Canada, retrieved 2014-02-02.