The other day I was reading something on Hackernews and someone posted a link to a Sexy Primes wikipedia article. I looked at that and then decided to do this in SQL Server because.. why not? Then I decided to see how different this would be to do in PostgreSQL. For the first method to create the prime numbers it's different. For the method with the CTE it is very similar
From the Sexy Primes wikipedia link: https://en.wikipedia.org/wiki/Sexy_prime
In mathematics, sexy primes are prime numbers that differ from each other by six. For example, the numbers 5 and 11 are both sexy primes, because 11 minus 5 is 6.
The term "sexy prime" is a pun stemming from the Latin word for six: sex.
If p + 2 or p + 4 (where p is the lower prime) is also prime, then the sexy prime is part of a prime triplet.
Ok I did a couple of versions of this and over the weekend.. here is what I ended up with
So first we need a table that will just have the prime numbers
I decided to populate a table with numbers from 2 till 500 and then use the sieve of Eratosthenes method to delete the non primes
This will look like this
Create this table
CREATE TABLE PrimeNumbers (N INT);
In one window run this to create the function/proc
CREATE OR REPLACE FUNCTION MakePrime() RETURNS void AS $$ DECLARE I integer := 2; BEGIN WHILE I <= SQRT(500) LOOP DELETE FROM PrimeNumbers WHERE N % I = 0 AND N > I; I := I + 1 ; END LOOP; END; $$ LANGUAGE plpgsql;
Thinking about it a little more I decided to do it with a CTE instead of a loop with delete statements, if your tables will be big then the delete method is probably better... it's for you to test that out :-)In a another window populate the table by making the call to the functionINSERT INTO PrimeNumbers(n) SELECT N FROM (SELECT generate_series(2,500) as n) x; SELECT MakePrime() ; -- Yes that is a proc call SELECT * FROM PrimeNumbers
What we are doing is a NOT EXISTS query against the same cte and we are filtering out numbers that are greater than the number in the current row and are not divisible by the current number
CREATE TABLE IF NOT EXISTS PrimeNumbers (N INT); ;WITH cte AS ( SELECT * FROM generate_series( 2, 500 ) n ) INSERT INTO PrimeNumbers SELECT n FROM cte WHERE NOT EXISTS ( SELECT n FROM cte as cte2 WHERE cte.n > cte2.n AND cte.n % cte2.n = 0) ; SELECT * FROM PrimeNumbers;
If we run that last select statement, we should have 95 rows
2
3
5
7
.....
.....
463
467
479
487
491
499
Now that we have our table filled with prime numbers till 500, it's time to run the queries
Sexy prime pairs
The sexy primes (sequences OEIS: A023201 and OEIS: A046117 in OEIS) below 500 are:
(5,11), (7,13), (11,17), (13,19), (17,23), (23,29), (31,37), (37,43), (41,47), (47,53), (53,59), (61,67), (67,73), (73,79), (83,89), (97,103), (101,107), (103,109), (107,113), (131,137), (151,157), (157,163), (167,173), (173,179), (191,197), (193,199), (223,229), (227,233), (233,239), (251,257), (257,263), (263,269), (271,277), (277,283), (307,313), (311,317), (331,337), (347,353), (353,359), (367,373), (373,379), (383,389), (433,439), (443,449), (457,463), (461,467).
Here is that query for the sexy prime pairs
-- 46 rows.. sexy primes SELECT t1.N,t2.N FROM PrimeNumbers t1 join PrimeNumbers t2 on t2.N - t1.N = 6 order by 1
It's very simple.. a self join that returns rows where the number from one table alias and the number from the other table alias differ by 6Prime triplets
The first prime triplets below 500 (sequence A098420 in the OEIS) are
(5, 7, 11), (7, 11, 13), (11, 13, 17), (13, 17, 19), (17, 19, 23), (37, 41, 43), (41, 43, 47), (67, 71, 73), (97, 101, 103), (101, 103, 107), (103, 107, 109), (107, 109, 113), (191, 193, 197), (193, 197, 199), (223, 227, 229), (227, 229, 233), (277, 281, 283), (307, 311, 313), (311, 313, 317), (347, 349, 353), (457, 461, 463), (461, 463, 467)
A prime triplet contains a pair of twin primes (p and p + 2, or p + 4 and p + 6), a pair of cousin primes (p and p + 4, or p + 2 and p + 6), and a pair of sexy primes (p and p + 6).
So we need to check that the 1st and 3rd number have a difference of 6, we also check that that difference between number 1 and 2 is 2 or 4. That query looks like this
-- 22 rows.. Prime Triplets SELECT t1.N AS N1,t2.N AS N2, t3.N AS N3 FROM PrimeNumbers t1 join PrimeNumbers t2 on t2.N > t1.N join PrimeNumbers t3 on t3.N - t1.N = 6 and t3.N > t2.N and t2.n - t1.n IN (2,4) order by 1Here is what it looks like from pgAdmin
Sexy prime triplets
Triplets of primes (p, p + 6, p + 12) such that p + 18 is composite are called sexy prime. p p, p+6 and p+12 are all prime, but p+18 is not
Those below 500 (sequence OEIS: A046118) are:
(7,13,19), (17,23,29), (31,37,43), (47,53,59), (67,73,79), (97,103,109), (101,107,113), (151,157,163), (167,173,179), (227,233,239), (257,263,269), (271,277,283), (347,353,359), (367,373,379)
The query looks like this.. instead of a self join, we do a triple self join, we also check that p + 18 is not a prime number in the line before the order by
-- 14 rows.. Sexy prime tripletsSELECT t1.N AS N1,t2.N AS N2, t3.N AS N3 FROM PrimeNumbers t1 join PrimeNumbers t2 on t2.n - t1.n = 6 join PrimeNumbers t3 on t3.N - t1.N = 12 and t3.N > t2.N AND NOT EXISTS( SELECT null FROM PrimeNumbers p WHERE p.n = t1.n +18) order by 1
And that's it for this post. If you are interested in the SQl Server version, you can find it here: Calculating Sexy Primes, Prime Triplets and Sexy Prime Triplets in SQL Server
More PostgreSQL posts can be found here: /label/PostgreSQL
