# 51 (number): Map

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51 (fifty-one) is the natural number 51 following 50 and preceding 52.

## In mathematics

Fifty-one is the 14th discrete biprime and the 5th in the {3.q} semiprime family having the prime factors (3.17). The aliquot sum of 51 is 21, itself a discrete biprime. 51 is the 4th member of the 11-aliquot tree with the sequence(51,21,11,1,0)

51 is a pentagonal number as well as a centered pentagonal number (one of the few numbers to be both) and an 18-gonal number and a Perrin 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.

Since the greatest prime factor of 512 + 1 = 2602 is 1301, which is clearly more than 51 twice, 51 is a Størmer number.

## In sports

• Las Vegas 51s, a minor league baseball team of the Pacific Coast League

## In other fields

Fifty-one is:

• Year identifier used on motor vehicles registered in the UK between 1 September 2001 and 28 February 2002

## Historical years

51 A.D., 51 B.C., 1951, 2051, etc.

## References

### Footnotes

1. [1] 51 (Orton) sqn ATC
2. 51 squadron history