255
votes
169
votes
Is this number a prime?
Hexagony, 29 bytes
.?'.).@@/'/.!.>+=(<.!)}($>(<%
The readable version of this code is:
...
- 3,351
135
votes
100
votes
Accepted
Find the Smoothest Number
CJam - 13
q~,>{mfW=}$0=
Try it at http://cjam.aditsu.net/
Example input: 2001 2014
Example output: ...
74
votes
Is this number a prime?
Hexagony, 218 92 58 55 bytes
Notice: This answer has been solidly beaten with a side-length 4 solution by Etoplay.
...
- 194k
72
votes
Find the Smoothest Number
Regex (.NET PCRE flavour), 183 129 bytes
Don't try this at home!
This is not really a contender for the win. But Eric Tressler suggested solving this problem with nothing but a regex, and I couldn't ...
- 194k
70
votes
65
votes
Is this number a prime?
Retina, 16 bytes
^(?!(..+)\1+$)..
Try it online!
Let's start with a classic: detecting primes with a regex. Input should be given in unary, using any repeated ...
- 194k
65
votes
Coprimes up to N
Python 2, 61 47 bytes
lambda n:[k/n for k in range(n*n)if k/n*k%n==1]
Try it online!
Background
Consider the ring \$(Z_n, +_n, \cdot_n)\$. While this ring is ...
- 207k
51
votes
Is this number a prime?
CJam, 4 bytes
qimp
CJam has a built-in operator for primality testing.
- 42.8k
51
votes
Is this number a prime?
Hexagony, 28 bytes
Since Etoplay absolutely trounced me on this question, I felt that I had to outgolf his only other answer.
...
- 11.4k
50
votes
Accepted
Is it almost-prime?
Sagemath, 2 bytes
GF
Outputs via exception.
Try it online!
The Sagemath builtin \$\text{GF}\$ creates a Galois Field of order \$n\$. However, remember that \$\...
- 12.9k
49
votes
49
votes
46
votes
Calculate Phi (not Pi)
Regex (.NET), 122 113 bytes
^(?=((?=.*$(?<=^(\3+(.+.))(.*?(?>(.\4)?)))).)+(.*))((?=.*(?=\6$)(?<=(?!(.+.)\8*(?=\6$)(?<=^\8+))(.+?(?>\9?)))).)+
...
39
votes
Accepted
Prime counting function
05AB1E, 3 bytes
!fg
This assumes that factorization built-ins are allowed. Try it online!
How it works
...
- 207k
38
votes
38
votes
Accepted
Determine whether a number is 2017-friable without primes in your source code
Jelly, 8 bytes
44‘²!*ḍ@
Try it online! Note that test cases 11111 and above are a bit too much for TIO.
Verification
...
- 207k
37
votes
Primality testing formula
43 bytes
(4**n+1)**n%4**n**2/n&2**(2*n*n+n)/-~2**n<1
Try it online!
The method is similar to Dennis' second (deleted) answer, but this answer is easier ...
- 17k
36
votes
Accepted
Prime Nerd Sniping Pattern
CJam, score 276496.9062958626
2,128*_(]128*
It turns out to be optimal, because: There aren't any non-orthogonal pairs with distance 2. So the distance must be ...
- 36.2k
34
votes
34
votes
Accepted
Output the Trinity Hall Prime
Jelly, 74 71 69 68 66 bytes
“©ạ-3ṗÇñ"ỤḍV8żṢ?ḤsMVE[,Ṃƭ"ḞÇsẇʂ(ụFsẠʂẆŀṣ’ḃ19ĖŒṙị⁾81s30m0Z062 ȷ446‘
Try it online!
How it works
The literal ...
- 207k
30
votes
Accepted
Longest Prime Sums
Score 100 8605
I used an algorithm that starts with one solution and repeatedly tries to split a prime \$p\$ in the solution into two other primes \$q_ 1\$ and \$q_ 2\$ that satisfy \$\frac1{p-1} = \...
- 14.9k
28
votes
Is this number a prime?
Brachylog (V2), 1 byte
ṗ
Try it online!
Brachylog (V1), 2 bytes
#p
This uses the built-in predicate #p - Prime, which ...
- 36.8k
28
votes
Is this number a prime?
Labyrinth, 29 bytes
1
?
:
} +{%!@
(:'(
} {
:**
Reads an integer from STDIN and outputs ((n-1)!)^2 mod n. Wilson's theorem ...
- 61.1k
28
votes
Is this number a prime?
Haskell, 49 bytes
Using xnor's Corollary to Wilson's Theorem:
main=do n<-readLn;print$mod(product[1..n-1]^2)n>0
- 66.2k
28
votes
Accepted
Calculate Phi (not Pi)
GS2, 12 10 bytes
V@'◄l.1&‼l
The source code uses the CP437 encoding. Try it online!
Test run
...
- 207k
28
votes
Compute the Carmichael function
Python, 76 73 67 bytes
f=lambda n,k=1:1-any(a**-~k*~-a**k%n for a in range(n))or-~f(n,k+1)
Try it online!
A further byte could be saved by returning True instead ...
- 207k
26
votes
Approximate Brun's Constant
Python 3, 78 77 75 70 68 62 bytes
f=lambda n,k=3,m=1,j=0:k<n and-m%k*j*2/k+f(n,k+2,m*k**4,m%k/k)
Thanks to @xnor for golfing off 2 4 bytes and paving the way ...
- 207k
Only top scored, non community-wiki answers of a minimum length are eligible