All of them should be prime

Is there exist an integer n , n, such that n 2 + 3 , n^2 +3, n 3 + 1 n^3+1 and n 4 + 5 n^4 + 5 are prime numbers?

No Yes

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3 solutions

Chan Tin Ping
Dec 25, 2017

Consider the expression n 3 + 1 n^3+1 , it can factorized to ( n + 1 ) ( n 2 n + 1 ) (n+1)(n^2-n+1) . Let n 3 + 1 n^3+1 is prime number, either ( n + 1 ) (n+1) or ( n 2 n + 1 ) (n^2-n+1) is 1 1 .

Now, if n + 1 = 1 n+1=1 , then n = 0 n=0 , which means n 3 + 1 = 1 n^3+1=1 which is not a prime nunber. If n 2 n + 1 = 1 n^2-n+1=1 , then n = 0 n=0 or n = 1 n=1 , which means that n 4 + 5 = 21 n^4+5=21 (n=1). Hence, the answer is no.

Anirudh Sreekumar
Dec 25, 2017

Since n is an integer, n 0 , 1 or 2 ( m o d 3 ) C a s e : 1 n 0 ( m o d 3 ) n 0 ( m o d 3 ) n 2 + 3 0 ( m o d 3 ) n 2 + 3 is not prime n = 0 is not valid as it implies n 3 + 1 = 1 which is not prime. C a s e : 2 n 1 ( m o d 3 ) n 1 ( m o d 3 ) n 4 + 5 0 ( m o d 3 ) n 4 + 5 is not prime C a s e : 3 n 2 ( m o d 3 ) We have, n 2 x 1 ( m o d 3 ) n 2 x + 1 2 ( m o d 3 ) n 4 1 ( m o d 3 ) n 4 + 5 0 ( m o d 3 ) n 4 + 5 is not prime At least one of n 2 + 3 , n 3 + 1 , n 4 + 5 will be composite for all n \begin{aligned}\text{Since }&\text{n is an integer,}\\ n&\equiv 0,1\text{ or }2\pmod{3}\\\\ \underline{Case:1 \hspace{4mm}\color{#3D99F6} n\equiv 0\pmod{3}}\\ n&\equiv 0\pmod{3}\\ \implies n^2+3&\equiv 0\pmod{3}\\ \implies n^2+3 &\text{ is not prime }\small \color{#3D99F6}\hspace{4mm} n=0 \text{ is not valid as it implies }n^3+1=1 \text{ which is not prime.}\\\\ \underline{Case:2 \hspace{4mm}\color{#3D99F6} n\equiv 1\pmod{3}}\\ n&\equiv 1\pmod{3}\\ \implies n^4+5&\equiv 0\pmod{3}\\ \implies n^4+5 &\text{ is not prime }\\\\ \underline{Case:3 \hspace{4mm}\color{#3D99F6} n\equiv 2\pmod{3}}\\ \text{We have,}\\ n^{2x}&\equiv 1\pmod{3}\\ n^{2x+1}&\equiv 2\pmod{3}\\ \implies n^4&\equiv 1\pmod{3}\\ \implies n^4+5&\equiv 0\pmod{3}\\ \implies n^4+5 &\text{ is not prime }\\\\ \implies \text{At least one of } &n^2+3,n^3+1,n^4+5 \text{ will be composite for all } n \end{aligned}

Cantdo Math
Apr 21, 2020

take n 4 + 5 n^4+5 modulo 3.If,n is not divisible by 3 then this is 0 (mod 3).Again,if n is divisible by 3,then the first term is divisible by 3.For that to be prime n should be 0,which contradicts that n^3+1 is a prime.

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