Find out the number of prime numbers

Let n = 2019 ! n = 2019! . Find the number of prime numbers that exist between n + 2 n + 2 to n + 2019 n + 2019 .

1 0 3 2 5 4

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2 solutions

Jordan Cahn
Jan 24, 2019

Interesting extension: There are 22 22 known n n for which n ! + 1 n!+1 is prime ( 2019 2019 is not one of them). It is conjectured (but still an open question) that there are infinite such n n .

See Factorial Primes

Agni Purani
Jan 24, 2019

If we take a general case: n + k n + k , where k = 2 , 3 , 4...2019 k = 2,3,4 ... 2019

n = 2019 ! = 2019 ( 2018 ) . . . . . ( k ) . . . . ( 2 ) ( 1 ) n = 2019! = 2019(2018).....(k)....(2)(1)

\therefore we can express n + k n + k as k ( 2019 ! k + 1 ) k(\frac{2019!}{k} + 1) , where k = 2 , 3 , . . 2019 k = 2,3,..2019 which is divisible by k k . So no prime numbers exist between n + 2 n + 2 and n + 2019 n + 2019 .

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