28 is a composite (non-prime) number whose divisors are 1, 2, 4, 7, 14, and of course 28.

28 is the second perfect number, the first being 6 and the third being 496 (with I am dubious about the prospects of living too). A perfect number is one which is equal to the sum of it's proper positive divisors, specifically 1+2+4+7+14=28.

Since 28

^{2}+1=785, and the largest prime factor of 785 is 157, which is more than 2x28, 28 is a StÃ¸rmer number.

28 is a harmonic divisor number, since the harmonic mean of it's divisors is an integer. Specifically: 6/(1+1/2+1/4+1/7+1/14+1/28)=3.

28 is a happy number, meaning that the sum of the squares of it's digits will recursively reduce to 1, like so:

2

^{2}+8

^{2}=68

6

^{2}+8

^{2}=100

1

^{2}=1

28 is a triangular number (meaning you can stack a 2D triangle with 28 items), a hexagonal number (meaning you can make a regular hexagon with 28 items), and a centered nonagonal number (meaning you can make a regular nine-sided polygon, or nonagon, with 28 items).

28 is a member of the Padovan sequence, which consists of the sum of the second and third preceeding members of the sequence, with the first three members defined as 1. Specifically: 1, 1, 1, 2, 2, 3, 4, 5, 7, 9, 12, 16, 21, 28...

28 is a Keith number, which means it appears in a Fibonacci-like sequence which starts with it's own digits. Specifically: 2, 8, 10, 18, 28...

28 is the atomic number for nickel.

28 is the fourth magic number, meaning that an atomic element with 28 nucleons will be more stable and resistant to radioactive decay.

28! = 304888344611713860501504000000

Messier object 28 is a globular cluster in the constellation Sagittarius approximately 19,000 light years away, and was the first globular cluster discovered to contain a millisecond pulsar.

NGC 28 is an elliptical galaxy located in the Phoenix constellation.

Texas was the 28th state to join the United States.

Woodrow Wilson was the 28th President of the United States, serving in that capacity during World War I.

The 28th Precinct in New York City is a largely residential community in Harlem.

The 28th episode of the Twilight Zone was "A Nice Place to Visit", which is about a gangster that dies and thinks he's gone to Heaven.

*Iron Man*v1 #28 was published in 1970. This is the 2nd panel on the 8th story page:

This is all good, but next year I get to be a

**prime**!

## 3 comments:

Ummm.... happy birthday?

Happy Birthday, Old Man!! Even I'm not 28! (Well, not until November, at least!)

Happy belated birthday.

Oddly enough, my wife to be had her 28th yesterday too.

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