Four consecutive odd numbers

Four consecutive odd numbers can never be primes

True or False?

Bonus : prove it.

False True

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2 solutions

Zakir Husain
Jun 21, 2020

Let there be an odd prime x x

Case 1 x 0 ( m o d 3 ) x\equiv0(\mod3)

Either x = 3 x=3 or it is composite, as it must be prime x = 3 \therefore x=3

Other odd numbers are 5 , 7 , 9 5,7,\red{9}

And 9 9 is not a prime

Case 2 x 1 ( m o d 3 ) x \equiv 1 (\mod 3)

x + 2 3 ( m o d 3 ) x + 2 0 ( m o d 3 ) 3 ( x + 2 ) x+2\equiv3 (\mod 3)\Rightarrow x+2\equiv0(\mod 3)\Rightarrow 3|(x+2)

Therefore either x + 2 = 3 x+2=3 or itis not a prime. If x + 2 = 3 x = 1 x+2=3\Rightarrow x=1 which is not a prime

Case 3 x 2 ( m o d 3 ) x \equiv 2 (\mod3)

x + 2 4 ( m o d 3 ) x+2\equiv4(\mod3)

x + 4 6 ( m o d 3 ) x + 4 0 ( m o d 3 ) 3 ( x + 4 ) x+4\equiv6(\mod3)\Rightarrow x+4\equiv 0(\mod3)\Rightarrow 3|(x+4)

Therefore either x + 4 = 3 x+4=3 or it is not a prime. If x + 4 = 3 x = 1 x+4=3\Rightarrow x=-1 which is not a prime

\therefore Four consecutive odd numbers can never be primes

Four consecutive odd numbers are such that, odd , even, odd , even, odd , even, odd

They are a total of seven consecutive numbers, since 3 is not divisible by two, it'll not repeatedly fall on even numbers, but on both odd and even alternatively, in seven, 3 must appear twice (3*2=6<7), and both can't be even, so one of four consecutive odd numbers is definitely a multiple of 3

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