## Happy 2019!

Happy 2019, the first 4 digit number to appear 6 times in the decimal expansion of Pi.

By the way:

2019 = 1^{4} + 2^{4} + 3^{4} + 5^{4} + 6^{4}.

Also, 2019 is the product of two primes 3 and 673. The sum of these two prime factors is a square.

This is not all that is interesting about factors of 2019. Every concatenation of these two prime factors is prime. Even more unusual, 2019 is the largest known composite number such that every concatenation of its prime factors is prime. [Oops, the last statement is wrong, Jan 3,2019]

Happy Happy-go-Lucky year, as 2019 is a Happy-go-Lucky number: the number that is both Happy and Lucky.

In case you are wondering, here is the definition of Happy numbers: One can take the sum of the squares of the digits of a number. Those numbers are Happy for which iterating this operation eventually leads to 1.

In case you are wondering, to build the Lucky number sequence, start with natural numbers. Delete every second number, leaving 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, …. The second number remaining is 3, so delete every third number, leaving 1, 3, 7, 9, 13, 15, 19, 21, …. The next number remaining is 7, so delete every 7th number, leaving 1, 3, 7, 9, 13, 15, 21, …. The next number remaining is 9, so delete every ninth number, etc.

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## Angelo Scordo:

The number 2019 can be represented as the sum of three squares in 7 different ways as follows:

2 January 2019, 4:21 pm1^2+13^2+43^2 = 2019

5^2+25^2+37^2 = 2019

7^2+11^2+43^2 = 2019

7^2+17^2+41^2 = 2019

11^2+23^2+37^2 = 2019

13^2+25^2+35^2 = 2019

17^2+19^2+37^2 = 2019

## Angelo Scordo:

If you are curious about the complete list of the 2019 number attributes have a look to this site:

3 January 2019, 3:33 amhttp://www.numbersaplenty.com/

This site is owned by an Italian computer scientist Giovanni Resta.

You can use the site for every number you are curious about up to 15 digits.

## Giovanni Resta:

“2019 is the largest known composite number such that every concatenation of its prime factors is prime.” does not seem to be right. Just take 2047 = 23 * 89 or 2059 = 29 * 71, and so on.

3 January 2019, 4:09 am## tanyakh:

Giovanni, thanks. I added a note.

3 January 2019, 10:07 am## Angelo Scordo:

A curiosity about the date:

4 January 2019, 3:14 amIf you compare all the dates of january today being 201914, searching for their appearance in the decimal expansion of Pi up to 40 million digits

you will find that all dates appear from 2 to 47 times but 2019125.

The today date appear 47 times as the date of tomorrow.

The first appearance of 201914 is between 404403 – 404407 digits.

## Angelo Scordo:

For accuracy sake, the date 2019/1/4 appears from the decimal position 404401 to 404406.

4 January 2019, 1:12 pm