Sieve of Sundaram
The Sieve of Sundaram is a sieve for a range of odd prime numbers.The sieve starts out listing the natural numbers to like the following example ()
Then it finds all numbers of form ( fancy way of saying start at and increase by , for each not already eliminated ) and eliminates them (empty cells):
Double each number and add one:
The basis of the algorithm is that for (without loss of generality, as the form we double and add 1 to is made up of commutative operations); with both factors nontrivial.
The equivalent in Step 2 above, of the Sieve of Eratosthenes starting the at the next prime squared, is starting at for next index
Generalization for prime finding
Looking close, if you know modular arithmetic, you'll note the remainder 1 forms the modular multiplicative group modulo 2. This generalizes to the modular multiplicative group mod any natural number( with loss of factors in the prime list). This generalization either needs copies of the same numbers ( one for each class), or (triangle numbers) colors . The class of remainder 1, is the only class where variable must stay nonzero to introduce a nontrivial factor by default ( negative representatives increase variable value required by 1). Modulo 30 is sketched out below (8 classes):
The table leaves off each to save space. the general rule, for modulus , with remainders is: in general.
Generalization for r-almost prime finding
- A natural number with prime factors including repetitions.
The sieve can be recursed on arguments and to check or for primality, if both are, we have a semiprime.
On the chance we don't get back prime for more than one, we can test the components. This will find out if our original number had 3 prime factors.
In the case of neither of get back prime we have at least 4 prime factors, and this can go to any depth you choose.