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Old 2020-11-13, 16:01   #1
rogue
 
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Default x!^n+/-1 or x#^n+/-1?

Is there any interest from anyone here in searching for primes in the form x!^n+/-1 or x#^n+/-1? I would refer to them as "power factorial" or "power primorial". As far as I know these forms have never been searched or n>1. It would be fairly easy to modify fsieve and psieve to support them.
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Old 2020-11-13, 16:39   #2
Dr Sardonicus
 
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Quote:
Originally Posted by rogue View Post
Is there any interest from anyone here in searching for primes in the form x!^n+/-1 or x#^n+/-1? I would refer to them as "power factorial" or "power primorial". As far as I know these forms have never been searched or n>1. It would be fairly easy to modify fsieve and psieve to support them.
Presumably you mean

\Phi_{n}(x!)\text{, }\Phi_{2n}(x!)\text{,}

or the same polynomials with the argument x! replaced with x#?
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Old 2020-11-13, 17:55   #3
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Quote:
Originally Posted by Dr Sardonicus View Post
Presumably you mean

\Phi_{n}(x!)\text{, }\Phi_{2n}(x!)\text{,}

or the same polynomials with the argument x! replaced with x#?
I'm not fully understanding your notation.

5! = 5*4*3*2*1 so 5!^3 = (5*4*3*2*1)^3
5# = 5*3*2 so 5#^3 = (5*3*2)^3

Here is a list of factorial primes: https://primes.utm.edu/top20/page.php?id=30
Here is a list of primorial primes: https://primes.utm.edu/top20/page.php?id=5.
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Old 2020-11-13, 18:02   #4
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Quote:
Originally Posted by rogue View Post
I'm not fully understanding your notation.
x^n+/-1 has trivial algebraic factors. So he was suggesting to look at the nth cyclotomic polynomial instead - i.e. what is left after removing the algebraic factors.
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Old 2020-11-13, 18:44   #5
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Quote:
Originally Posted by axn View Post
x^n+/-1 has trivial algebraic factors. So he was suggesting to look at the nth cyclotomic polynomial instead - i.e. what is left after removing the algebraic factors.
Exactly. There is a not hard pattern for these factors: if additionally you remove all lower exponents factors then for the remaining p prime factors what divides polcyclo(n,x) we have p%n=1. You really need to do this, because for example:
Code:
? factor(polcyclo(20,2))
%24 = 
[ 5 1]

[41 1]
Ofcourse after the sieve with these special primes you need to do only a few gcd's on the remaining candidates. [or start the sieve removing these].

Last fiddled with by R. Gerbicz on 2020-11-13 at 18:46 Reason: grammar
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Old 2020-11-13, 19:40   #6
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As for algebraic factors, I had forgotten about those.
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Old 2020-11-13, 20:16   #7
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Doing a quick search with pfgw with the -f1 flag, the following numbers are prime or 3-PRP for the form x!^2 +/- 1 up to x = 2000:

Code:
0!^2+1
1!^2+1
2!^2+1
3!^2+1
4!^2+1
5!^2+1
9!^2+1
10!^2+1
11!^2+1
13!^2+1
24!^2+1
65!^2+1
76!^2+1
2!^2-1
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Old 2020-11-13, 20:59   #8
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Quote:
Originally Posted by Dylan14 View Post
Doing a quick search with pfgw with the -f1 flag, the following numbers are prime or 3-PRP for the form x!^2 +/- 1 up to x = 2000:

Code:
0!^2+1
1!^2+1
2!^2+1
3!^2+1
4!^2+1
5!^2+1
9!^2+1
10!^2+1
11!^2+1
13!^2+1
24!^2+1
65!^2+1
76!^2+1
2!^2-1
I wouldn't expect anything else on the -1 side as x^n-1 always factors as (x-1) as a factor. Note that on the +1 side any primes must also be GFNs which means that n will always be a power of 2 for primes.

So from those perspectives, these forms are not that interesting.
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Old 2020-11-14, 11:51   #9
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Remove the +/-1 and add +/-k and divide by k where k | x!^n

Things will get interesting Ali candidates will be proveable via N-/+1 method since they will be of form where N has prime factors equal to or less than x.
Additionally if both (x!^n+/-k)/k are prime then they are twin primes.
Here is non twin, n=1 example:

https://www.mersenneforum.org/showpo...8&postcount=19

ETA Unlike non-generalized factorial primes there are plenty of twin-primes in the generated form which are highly unreserved.

Last fiddled with by a1call on 2020-11-14 at 12:28
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Old 2020-11-14, 13:58   #10
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Quote:
Originally Posted by R. Gerbicz View Post
Exactly. There is a not hard pattern for these factors: if additionally you remove all lower exponents factors then for the remaining p prime factors what divides polcyclo(n,x) we have p%n=1. You really need to do this, because for example:
Code:
? factor(polcyclo(20,2))
%24 = 
[ 5 1]

[41 1]
Ofcourse after the sieve with these special primes you need to do only a few gcd's on the remaining candidates. [or start the sieve removing these].
Right, the factor 5 in the above is sometimes called an "intrinsic" prime factor. It shows up because 5 divides polcyclo(4,2). An additional factor of 5 shows up in polcyclo(4*5, 2); another in polcyclo(4*5^2, 2), another in polcyclo(4*5^3, 2) and so on. This is described in detail in The Cyclotomic Polynomials.

There is at most one "intrinsic" prime factor. Let P be the evaluation of the cyclotomic polynomial, and n the exponent. If P has an intrinsic prime factor, it is gcd(P,n).
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