Is there exist an integer n , such that n 2 + 3 , n 3 + 1 and n 4 + 5 are prime numbers?
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Since n C a s e : 1 n ≡ 0 ( m o d 3 ) n ⟹ n 2 + 3 ⟹ n 2 + 3 C a s e : 2 n ≡ 1 ( m o d 3 ) n ⟹ n 4 + 5 ⟹ n 4 + 5 C a s e : 3 n ≡ 2 ( m o d 3 ) We have, n 2 x n 2 x + 1 ⟹ n 4 ⟹ n 4 + 5 ⟹ n 4 + 5 ⟹ At least one of n is an integer, ≡ 0 , 1 or 2 ( m o d 3 ) ≡ 0 ( m o d 3 ) ≡ 0 ( m o d 3 ) is not prime n = 0 is not valid as it implies n 3 + 1 = 1 which is not prime. ≡ 1 ( m o d 3 ) ≡ 0 ( m o d 3 ) is not prime ≡ 1 ( m o d 3 ) ≡ 2 ( m o d 3 ) ≡ 1 ( m o d 3 ) ≡ 0 ( m o d 3 ) is not prime n 2 + 3 , n 3 + 1 , n 4 + 5 will be composite for all n
take 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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Consider the expression n 3 + 1 , it can factorized to ( n + 1 ) ( n 2 − n + 1 ) . Let n 3 + 1 is prime number, either ( n + 1 ) or ( n 2 − n + 1 ) is 1 .
Now, if n + 1 = 1 , then n = 0 , which means n 3 + 1 = 1 which is not a prime nunber. If n 2 − n + 1 = 1 , then n = 0 or n = 1 , which means that n 4 + 5 = 2 1 (n=1). Hence, the answer is no.