51 (number)

51 is the natural number following 50 and preceding 52.

List of numbers — Integers << 50 51 52 53 54 55 56 57 58 59 >>
Cardinal	fifty-one
Ordinal	51st (fifty-first)
Factorization	<math>3\cdot 17<math>
Roman numeral	LI
Binary	110011
Hexadecimal	33

Fifty-one is a pentagonal number as well as a centered pentagonal number (one of the few numbers to be both) and an octodecagonal number. It is also the 6th Motzkin number, telling the number of ways to draw non-intersecting chords between any six points on a circle's boundary, no matter where the points may be located on the boundary.

It is also:

• the atomic number of antimony.
• the code for international direct dial phone calls to Peru.
• the model number of the P-51 Mustang WWII fighter aircraft
• part of the title of a collection of stories by Lord Dunsany, Fifty-One Tales.
• Area 51, a parcel of U.S. military-controlled land in southern Nevada, apparently containing a secret aircraft testing facility.
• The year AD 51, 51 BC, and 1951.