# Prime Modified Z-Factorials

Let me explain one by one the above terms...

We will call $$\\text{Z-Factorial}(n)\$$ of a positive integer $$\n\$$, $$\n!\$$ (i.e. $$\n\$$ factorial) without any trailing zeros. So, $$\\text{Z-Factorial}(30)\$$ is $$\26525285981219105863630848\$$ because $$\30!=265252859812191058636308480000000\$$

We will call Modified Z-Factorial of $$\n\$$, the $$\\text{Z-Factorial}(n) \mod n\$$.
So, Modified Z-Factorial of $$\30\$$, is $$\\text{Z-Factorial}(30) \mod 30\$$ which is $$\26525285981219105863630848 \mod 30 = 18\$$

We are interested in those $$\n\$$'s for which the Modified Z-Factorial of n is a Prime Number

Example

The number $$\545\$$ is PMZ because $$\\text{Z-Factorial}(545) \mod 545 = 109\$$ which is prime

Here is a list of the first values of $$\n\$$ that produce Prime Modified Z-Factorial (PMZ)

5,15,35,85,545,755,815,1135,1165,1355,1535,1585,1745,1895,1985,2005,2195,2495,2525,2545,2615,2705,2825,2855,3035,3085,3155,3205,3265,3545,3595,3695,3985,4135,4315,4385,4415,4685,4705,4985,5105,5465,5965,6085,6155,6185,6385,6415,6595...


The above list goes on and your task is to find the $$\k\$$th PMZ

Input

A positive integer $$\k\$$

Output

The $$\kth\$$ PMZ

Test Cases

here are some 1-indexed test cases.
Your solutions need only work within the bounds of your language's native integer size.

input -> output
1        5
10       1355
21       2615
42       5465
55       7265
100      15935
500      84815


This is , so the lowest score in bytes wins.

• I think we have Z-Factorial(755) mod 755 = 151, which is prime. Yet it's not included in your list. Am I missing something? Aug 17, 2020 at 17:59
• You are right! Let me fix this... Aug 17, 2020 at 18:02
• Do you mind me adding in some MathJax to the question body? It's slightly hard to read with all the code blocks Aug 17, 2020 at 18:19
• @cairdcoinheringaahing be my guest! Aug 17, 2020 at 18:20
• @Shaggy Absolutely yes Aug 17, 2020 at 22:51

# 05AB1E, 16 bytes

[N!0ÜN%pi®>©¹Q#N


Input is 1-based k.

Outputs the k-th PMZ.

Explanation:

[N!0ÜN%pi®>©¹Q#N
[                     Start infinite loop
N!                   Factorial of the index
0Ü                 Remove trailing zeros
N%               Mod index
p              Is prime?
i             If it is:
®>©          Increment the value stored in register c (initially -1)
¹Q        Is the value equals the input?
#N      If it does, push the index (which is the PMZ) and break


Try it online!

• Seems like you’re not aware of the µ built-in. 9 bytes: µNN!0ÜN%p Aug 18, 2020 at 17:34
• How did I missed that?! Thank you.. Should I add it to this answer? Aug 18, 2020 at 17:57
• Yep feel free to edit it in! Aug 19, 2020 at 2:44

# Jelly,  13  11 bytes

!Dt0Ḍ%⁸Ẓµ#Ṫ


A full program reading from STDIN which prints the result to STDOUT.

Try it online!

### How?

!Dt0Ḍ%⁸Ẓµ#Ṫ - Main Link: no arguments
#  - set n=0 (implicit left arg) and increment getting the first
(implicit input) values of n which are truthy under:
µ   -   the monadic chain (f(n)):
!           -     factorial -> n!
D          -     convert from integer to decimal digits
t0        -     trim zeros
Ḍ       -     convert from decimal digits to integer
⁸     -     chain's left argument, n
%      -     modulo
Ẓ    -     is prime?
Ṫ - tail
- implicit print

• @FryAmTheEggman Ah, I thought I could remove the ³. Aug 17, 2020 at 18:42
• @FryAmTheEggman ...turns out I can if I use a link rather than a chain, so back to 13 again :) Aug 17, 2020 at 18:45
• Nice! When you are done golfing I'm really curious what the 5 is doing. I only know Jelly mildly and it seems quite odd (removing it sometimes causes off by one errors?). Aug 17, 2020 at 19:02
• @FryAmTheEggman 5 is the starting value of the incremental search which otherwise defaults to the input (so we find the first n integers starting at 5 counting up that satisfy the property rather than the first n integers starting at n) - the Ṫ just gives us the last one. EDIT: Although saying that we can get rid of it by using STDIN... Aug 17, 2020 at 19:06
• Not the most usual method of golf help, but if it works it works :) I was wondering why it wasn't beating my test program in Pyth by more since you had access to trim, now it seems much cleaner! Aug 17, 2020 at 19:17

D,f,@,Rb*BDBGbUdb*!!*BFJiA%P
x:?
Wx,y,+1,z,$f>y,x,-z Oy  Try it online! Times outs for $$\k \ge 30\$$ on TIO ## How it works D,f,@, ; Define a function, f, taking 1 argument, n ; Example: STACK =  Rb* ; Factorial STACK =  BD ; Convert to digits STACK = [2 6 5 ... 0 0 0] BGbU ; Group adjacents STACK = [   ...    [0 0 0 0 0 0 0]] db*!! ; If last is all 0s *BF ; remove it STACK = [   ...   ] Ji ; Join to make integer STACK =  A% ; Mod n STACK =  P ; Is prime? STACK =  ; Return top value 0 x:? ; Set x to the input Wx, ; While x > 0 y,+1, ; y = y + 1 z,$f>y,	;	z = f(y)
x,-z		;	x = x - z
; We count up with y
; If y is PMZ, set z to 1 else 0
; Subtract z from x, to get x PMZs

Oy			; Output y


# Japt, 13 bytes

0-indexed. Only works, in practice, for 0 & 1 as once we go over 21! we exceed JavaScript's MAX_SAFE_INTEGER.

ÈÊsÔsÔuX j}iU


Try it

ÈÊsÔsÔuX j}iU     :Implicit input of integer U
È                 :Function taking an integer X as argument
Ê                :  Factorial
s               :  String representation
Ô              :    Reverse
sÔ            :  Repeat (There has to be a shorter way to remove the trailing 0s!)
uX          :  Modulo X
j        :  Is prime?
}       :End function
iU     :Pass all integers through that function, returning the Uth one that returns true


# R, 99 93 bytes

Edit: -6 bytes (and -4 bytes from arbitrary-precision version) thanks to Giuseppe

k=scan();while(k){F=F+1;z=gamma(F+1);while(!z%%5)z=z/10;x=z%%F;k=k-(x==2|all(x%%(2:x^.5)))};F


Try it online!

Uses the straightforward approach, following the steps of the explanation. Unfortunately goes out of limits of R's numerical accuracy at factorial(21), so fails for any k>2.

An arbitrary-precision version (which is not limited to small k, but is less golf-competitive) is:
R + gmp, 115 bytes

• 93 bytes. Similar golfs can (probably) be applied to the gmp approach Aug 18, 2020 at 19:39
• Thanks Giuseppe! Aug 19, 2020 at 7:25

# Husk, 11 bytes

!foṗS%ȯ↔↔ΠN


Try it online!

## Explanation

!foṗS%ȯ↔↔ΠN
f        N filter list of natural numbers by:
Π  take factorial
↔↔   reverse twice, remove trailing zeros
S%     mod itself
ṗ       is prime?
!           get element at index n


# JavaScript (Node.js),  89 ... 79  77 bytes

n=>(g=y=>y%10n?(p=k=>y%--k?p(k):~-k||--n?g(x*=++i):i)(y%=i):g(y/10n))(x=i=2n)


Try it online!

• g=y (15 chars)
– null
Aug 18, 2020 at 9:12
• @HighlyRadioactive What do you mean? Aug 18, 2020 at 9:17
• A bad, dirty joke. Never mind.
– null
Aug 18, 2020 at 9:18

# Python 3, 145140138 129 bytes

def f(n,r=0):
c=d=2
while r<n:
c+=1;d*=c
while 1>d%10:d//=10
i=d%c;r+=i==2or i and min(i%j for j in range(2,i))
return c


Try it online!

# Python 2, 126 125 bytes

def f(n,r=0):
c=d=2
while r<n:
c+=1;d*=c
while d%10<1:d/=10
i=d%c
r+=i==2or i and min(i%j for j in range(2,i))
print c


Try it online!

Explanation: Keep dividing by 10 as long as the current factorial is divisible by 10, and then check the factorial modulo current number for primality.

Thanks to caird coinheringaahing for -20 bytes and Dominic van Essen for -9 bytes!

• 135 bytes by switching to Python 2, or 140 sticking with Python 3 Aug 17, 2020 at 20:22
• 125 bytes using Python 2 Aug 17, 2020 at 20:29
• thanks a lot ^^ Aug 17, 2020 at 20:33
• 129 bytes in Python 3 using ; and some of the Python 2 tricks... Aug 18, 2020 at 9:49

g n
|nmod10>0=n
|0<1=g$div n 10 f=(!!)[n|n<-[1..],let p=mod(g$product[1..n])n,[x|x<-[2..p],mod p x<1]==[p]]


Try it online!

g removes 0s from number.

f takes kth element from an infinite list comprehension where:
[x|x<-[2..p],mod p x==0]==[p] is prime condition( compares list of divisors of p and a list of just p).

And p is mod(g\$foldr(*)1[1..n])n the modulo of factorial passed through g.

Saved 18 thanks to user

• Wow, this is way more concise than the one I got. I think you can reduce it even more to 123 bytes
– user
Aug 18, 2020 at 16:59
• Make that 113
– user
Aug 18, 2020 at 17:03
• Thanks @user I don't know these tricks! Aug 18, 2020 at 19:04