# Is it almost-prime?

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Definition: A positive integer n is almost-prime, if it can be written in the form n=p^k where p is a prime and k is also a positive integers. In other words, the prime factorization of n contains only the same number.

Input: A positive integer 2<=n<=2^31-1

Output: a truthy value, if n is almost-prime, and a falsy value, if not.

Truthy Test Cases:

2
3
4
8
9
16
25
27
32
49
64
81
1331
2401
4913
6859
279841
531441
1173481
7890481
40353607
7528289


Falsy Test Cases

6
12
36
54
1938
5814
175560
9999999
17294403


Please do not use standard loopholes. This is so the shortest answer in bytes wins!

• To clarify: the truthy and falsy values need not be consistent, right? – Luis Mendo Aug 26 at 0:42
• This is A000961 in the OEIS. – Giuseppe Aug 26 at 13:21
• The usual name for this kind of number is "prime power". – Andreas Rejbrand Aug 26 at 17:39
• It feels odd to me that you include prime numbers as being "almost prime," but this is still a good challenge! :) – Captain Man Aug 26 at 18:26
• This should use the terminology "prime power". en.wikipedia.org/wiki/Almost_prime already has a definition. – qwr Aug 28 at 5:27

# Sagemath, 2 bytes

GF


Outputs via exception.

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The Sagemath builtin $$\\text{GF}\$$ creates a Galois Field of order $$\n\$$. However, remember that $$\\mathbb{F}_n\$$ is only a field if $$\n = p^k\$$ where $$\p\$$ is a prime and $$\k\$$ a positive integer. Thus the function throws an exception if and only if its input is not a prime power.

• Galois fields was the first thing I thought of, but I had no idea that Sagemath had a builtin for it. – Don Thousand Aug 26 at 14:30
• @DonThousand you should see what you can do with SM, it's support for Elliptic curve math/crypto is fantastic. – Woodstock Aug 26 at 17:37

# Python 2, 42 bytes

f=lambda n,p=2:n%p and f(n,p+1)or p**n%n<1


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Since Python doesn't have any built-ins for primes, we make do with checking divisibility.

We find the smallest prime p that's a factor of n by counting up p=2,3,4,... until n is divisible by p, that is n%p is zero. There, we check that this p is the only prime factor by checking that a high power of p is divisible by n. For this, p**n suffices.

As a program:

43 bytes

n=input()
p=2
while n%p:p+=1
print p**n%n<1


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This could be shorter with exit codes if those are allowed.

46 bytes

lambda n:all(n%p for p in range(2,n)if p**n%n)


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# Shakespeare Programming Language, 329 bytes

,.Ajax,.Page,.Act I:.Scene I:.[Enter Ajax and Page]
Ajax:Listen tothy.
Page:You cat.
Scene V:.
Page:You is the sum ofYou a cat.
Is the remainder of the quotient betweenI you nicer zero?If soLet usScene V.
Scene X:.
Page:You is the cube ofYou.Is you worse I?If soLet usScene X.
You is the remainder of the quotient betweenYou I.Open heart


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Outputs 0 if the input is almost prime, and a positive integer otherwise. I am not sure this is an acceptable output; changing it would cost a few bytes.

Explanation:

• Scene I: Page takes in input (call this n). Initialize Ajax = 1.
• Scene V: Increment Ajax until Ajax is a divisor of Page; call the final value p This gives the smallest divisor of Page, which is guaranteed to be a prime.
• Scene X: Cube Ajax until you end up with a power of p, say p^k which is greater than n. Then n is almost-prime iff n divides p^k.
• You can save a byte by removing the space after "the remainder of", right? – Hello Goodbye Aug 29 at 1:56
• @HelloGoodbye No, that space is needed, as the name of the function is the_remainder_of_the_quotient_between. If you remove the space in the middle of the name, the function is not applied. – Robin Ryder Aug 29 at 5:04
• Right, okay. I'd forgotten about that function – Hello Goodbye Aug 30 at 17:44

# MATL, 4 bytes

Yf&=

• For almost-primes the output is a matrix containing only 1s, which is truthy.
• Otherwise the output is a matrix containing several 1s and at least one 0, which is falsy.

Try it online! Or verify all test cases, including truthiness/falsihood test.

### How it works

     % Implicit input
Yf   % Prime factors. Gives a vector with the possibly repeated prime factors
&=   % Matrix of all pair-wise equality comparisons
% Implicit output


# R, 3632 29 bytes

-3 bytes by outputting a vector of booleans without extracting the first element

!(a=2:(n=scan()))[!n%%a]^n%%n


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Outputs a vector of booleans. In R, a vector of booleans is truthy iff the first element is TRUE.

First, find the smallest divisor p of n. We can do this by checking all integers (not only primes), as the smallest divisor of an integer (apart from 1) is always a prime number. Here, let a be all the integers between 2 and n, then p=a[!n%%a] is the first element of a which divides n.

Then n is almost prime iff n divides p^n.

This fails for any moderately large input, so here is the previous version which works for most larger inputs:

# R, 36 33 bytes

!log(n<-scan(),(a=2:n)[!n%%a])%%1


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Compute the logarithm of n in base p: this is an integer iff n is almost prime.

This will fail due to floating point inaccuracy for certain (but far from all) large-ish inputs, in particular for one test case: $$\4913=17^3\$$.

• Brilliant and 25 bytes less than my own best attempt without peeking... The log trick is super! – Dominic van Essen Aug 26 at 8:05
• Although a nice approach, I'm afraid it fails for test case 4913 due to floating point inaccuracies (2.9999999999999996 is not an integer). I've just looked in the meta, and apparently you have to work around this if your language supports an accurate decimal type. I don't know R, so I don't know if this applies to it, but I was about to port your approach to Java to golf my about to post answer, but that apparently wouldn't be allowed unless I use java.math.BigDecimal instead of regular doubles.. :/ – Kevin Cruijssen Aug 26 at 8:48
• @KevinCruijssen I assumed this was OK, as it would work with a theoretical infinite-precision computer. I am probably not aware of the meta consensus you referred to, could you link to it? – Robin Ryder Aug 26 at 9:06
• @RobinRyder Ah, I thought I added a link. Here it is. – Kevin Cruijssen Aug 26 at 9:09
• @DominicvanEssen While you were commenting, I made a similar change: I don't need …^(3*n) but simply …^n, which gains 4 bytes (but fails for any moderately large input). – Robin Ryder Aug 26 at 9:58

# C (gcc), 43 bytes

f(n,i){for(i=1;n%++i;);n=i<n&&f(n/i)^i?:i;}


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Returns p if n is almost-prime, and 1 otherwise.

f(n,i){
for(i=1;n%++i;);    // identify i = the least prime factor of n
n=i<n&&f(n/i)^i     // if n is neither prime nor almost-prime
?                 //  return 1
:i;               // return i
}


# Wolfram Language (Mathematica), 11 bytes

PrimePowerQ


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@Sisyphus saved 1 byte

# J, 9 8 bytes

1=#@=@q:


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-1 byte thanks to xash

Tests if the self-classify = of the prime factors q: has length # equal to one 1=

# 05AB1E, 2 bytes

ÒË


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### Commented:

Ò   -- Are all the primes in the prime decomposition
Ë  -- Equal?

• A pure ASCII alternative: fg - since only 1 is truthy in 05AB1E. – hi. Aug 26 at 2:02

# APL (Dyalog Classic), 3331 26 bytes

{⍵∊∊(((⊢~∘.×⍨)1↓⍳)⍵)∘*¨⍳⍵}


-5 bytes from Kevin Cruijssen's suggestion.

Warning: Very, very slow for larger numbers.

## Explanation

{⍵∊∊(((⊢~∘.×⍨)1↓⍳)⍵)∘*¨⍳⍵} ⍵=n in all the following steps
⍳⍵  range from 1 to n
∘*¨    distribute power operator across left and right args
(((⊢~∘.×⍨)1↓⍳)⍵)       list of primes till n
∊                       flatten the right arg(monadic ∊)
⍵∊                        is n present in the primes^(1..n)?


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• I'm not entirely sure, but I think you can drop the *0.5 and just use a range or 1 to n? – Kevin Cruijssen Aug 26 at 7:46
• It is way too slow with that but yeah, sure. – Razetime Aug 26 at 7:52
• Well, codegolf is all about saving bytes, so compilation warnings, code standards, and performance are all irrelevant in code-golfing. Even if the performance would go from $O(1)$ to $O(n^n)$, if we can save even a single byte it's worth it, haha. ;) But if TIO would be to slow to run, you could leave the *0.5 in the TIO and mention it's only used to speed up the online code. (PS: The 0.5 could have been golfed to .5 if it was indeed required.) – Kevin Cruijssen Aug 26 at 7:55

# Pyth, 5 bytes

!t{PQ


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Explanation:

Q - Takes integer input
P - List of prime factors
{ - Remove duplicate elements
t - Removes first element
! - Would return True if remaining list is empty, otherwise False

• I have absolutely no idea what I'm doing, but does this work? – Unrelated String Aug 26 at 3:48
• @UnrelatedString that does, but I am not sure if im allowed to do that since it returns 0 (falsy value) for right cases and a truthy value for the others (which isn't fixed either). – Manish Kundu Aug 26 at 3:51
• Well, in that case, !s.+PQ is still a byte shorter. – Unrelated String Aug 26 at 3:53

## Setanta, 61 59 bytes

gniomh(n){p:=2nuair-a n%p p+=1nuair-a n>1 n/=p toradh n==1}


Try it here

Notes:

• The proper keyword is gníomh, but Setanta allows spelling it without the accents so I did so to shave off a byte.
• What an interesting language! Makes me want to go try to learn Irish again, and Setanta, for that matter! – Giuseppe Aug 26 at 16:51

f n=mod(until((<1).mod n)(+1)2^n)n<1


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36 bytes

f n=and[mod(gcd d n^n)n<2|d<-[1..n]]


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39 bytes

f n=all((elem[1,n]).gcd n.(^n))[2..n]


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39 bytes

f n=mod n(n-sum[1|1<-gcd n<$>[1..n]])<1  Try it online! 40 bytes f n=and[mod(p^n)n<1|p<-[2..n],mod n p<1]  Try it online! # JavaScript (ES6), 43 bytes ## Without BigInts Returns a Boolean value. f=(n,k=1)=>n%1?!~~n:f(n<0?n/k:n%++k?n:-n,k)  Try it online! A recursive function that first looks for the smallest divisor $$\k>1\$$ of $$\n\$$ and then divides $$\-n\$$ by $$\k\$$ until it's not an integer anymore. (The only reason why we invert the sign of $$\n\$$ when $$\k\$$ is found is to distinguish between the two steps of the algorithm.) If $$\n\$$ is almost-prime, the final result is $$\-\dfrac{1}{k}>-1\$$. So we end up with $$\\lceil n\rceil=0\$$. If $$\n\$$ is not almost-prime, there exists some $$\q>k\$$ coprime with $$\k\$$ such that $$\n=q\times k^{m}\$$. In that case, the final result is $$\-\dfrac{q}{k}<-1\$$. So we end up with $$\\lceil n\rceil<0\$$. # JavaScript (ES11), 33 bytes ## With BigInts With BigInts, using @xnor's approach is probably the shortest way to go. Returns a Boolean value. f=(n,k=1n)=>n%++k?f(n,k):k**n%n<1  Try it online! # Retina 0.8.2, 50 bytes .+$*
^(?=(11+?)\1*$)((?=\1+$)(?=(1+)(\3+)$)\4)+1$


Try it online! Link includes faster test cases. Based on @Deadcode's answer to Match strings whose length is a fourth power. Explanation:

.+
$*  Convert the input to unary. ^(?=(11+?)\1*$)


Start by matching the smallest factor $$\ p \$$ of $$\ n \$$. ($$\ p \$$ is necessarily prime, of course.)

(?=\1+$)(?=(1+)(\3+)$)


While $$\ p | \frac n { p^i } \$$, find $$\ \frac n { p^i } \$$'s largest proper factor, which is necessarily $$\ \frac n { p^{i+1} } \$$.

\4


The factorisation also captures $$\ (p - 1) \frac n { p^{i+1} } \$$, which is subtracted from $$\ \frac n { p^i } \$$, leaving $$\ \frac n { p^{i+1} } \$$ for the next pass through the loop.

(...)+1$ Repeat the division by $$\ p \$$ as many times as possible, then check that $$\ \frac n { p^k } = 1 \$$. # Io, 48 bytes Port of @RobinRyder's R answer. method(i,c :=2;while(i%c>0,c=c+1);i log(c)%1==0)  Try it online! ## Explanation method(i, // Take an input c := 2 // Set counter to 2 while(i%c>0, // While the input doesn't divide counter: c=c+1 // Increment counter ) i log(c)%1==0 // Is the decimal part of input log counter equal to 0? )  # Assembly (MIPS, SPIM), 238 bytes, 6 * 23 = 138 assembled bytes main:li$v0,5
syscall
move$t3,$v0
li$a0,0 li$t2,2
w:bgt$t2,$t3,d
div$t3,$t2
mfhi$t0 bnez$t0,e
add$a0,$a0,1
s:div$t3,$t2
mfhi$t0 bnez$t0,e
div$t3,$t3,$t2 b s e:add$t2,$t2,1 b w d:move$t0,$a0 li$a0,0
bne$t0,1,p add$a0,$a0,1 p:li$v0,1
syscall


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• You can save bytes if you use $2 through$9, rather than named registers. – insou Sep 20 at 8:27

# Brachylog, 2 bytes

Are all prime factors equal?

ḋ=


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# GAP 4.7, 31 bytes

n->Length(Set(FactorsInt(n)))<2

This is a lambda. For example, the statement

Filtered([2..81], n->Length(Set(FactorsInt(n)))<2 );

yields the list [ 2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 17, 19, 23, 25, 27, 29, 31, 32, 37, 41, 43, 47, 49, 53, 59, 61, 64, 67, 71, 73, 79, 81 ].

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• Please link your answers to Try It Online or any other interpreter so they can be verified properly. – Razetime Aug 26 at 5:49

# MathGolf, 10 bytes

╒g¶mÉk╒#─╧


Port of @Razetime's APL (Dyalog Classic) answer, so make sure to upvote him as well!

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Explanation:

╒           # Push a list in the range [1, (implicit) input-integer)
g          # Filter it by:
¶         #  Check if it's a prime
m        # Map each prime to,
É       # using the following three operations:
k╒     #  Push a list in the range [1, input-integer) again
#    #  Take the current prime to the power of each value in this list
─   # After the map, flatten the list of lists
╧  # And check if this list contains the (implicit) input-integer
# (after which the entire stack joined together is output implicitly)


# Factor, 35 bytes

: f ( n -- ? ) factors all-equal? ;


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• @petStorm Here it is :) Thanks! – Galen Ivanov Aug 26 at 11:13

# Japt, 6 bytes

I feel like this should be 1 or 2 bytes shorter ...

k ä¶ ×


Try it - includes all test cases

# Java, 69 (or 64?) bytes

n->{int c=0,t=1;for(;t++<n;)if(n%t<1)for(c++;n%t<1;)n/=t;return c<2;}


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Explanation:

n->{                // Method with integer parameter and boolean return-type
int c=0,          //  Counter-integer, starting at 0
t=1;for(;t++<n;)  //  Loop t in the range (1,n]:
if(n%t<1)       //   If the input is divisible by t:
for(c++;      //    Increase the counter by 1
n%t<1;)   //    Loop as long as the input is still divisible by t
n/=t;       //     And divide n by t every iteration
return c<2;}      //  Return whether the counter is 1


If we would be allowed to ignore floating point inaccuracies, a port of @RobinRyder's R answer would be 64 bytes instead:

n->{int m=1;for(;n%++m>0;);return Math.log(n)/Math.log(m)%1==0;}


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Explanation:

n->{               // Method with integer parameter and boolean return-type
int m=1;         //  Minimum divisor integer m, starting at 1
for(;n%++m>0;);  //  Increase m by 1 before every iteration with ++m
//  And continue looping until the input is divisible by m
return Math.log(n)/Math.log(m)
//  Calculate log_m(n)
%1==0;}   //  And return whether it has no decimal values after the comma


But unfortunately this approach fails for test case 4913 which would become 2.9999999999999996 instead of 3.0 due to floating point inaccuracies (it succeeds for all other test cases).
A potential fix would be 71 bytes:

n->{int m=1;for(;n%++m>0;);return(Math.log(n)/Math.log(m)+1e9)%1<1e-8;}


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# Jelly, 3 bytes

ÆfE


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# Burlesque, 6 bytes

rifCsm


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Explanation:

ri      # Read integer from input
fC    # Find its prime factorisation
sm  # Are all values the same?