Sum and products of consecutive integers

1 × 2 1\times2 is not divisible by 1 + 2 1+2 .
1 × 2 × 3 × 4 1\times2\times3\times4 is not divisible by 1 + 2 + 3 + 4 1+2+3+4 .
1 × 2 × 3 × 4 × 5 × 6 1\times2\times3\times4\times5\times6 is not divisible by 1 + 2 + 3 + 4 + 5 + 6 1+2+3+4+5+6 .

Is it true that there are infinitely many even positive integers m m such that 1 × 2 × × m 1\times2\times\cdots\times m is not divisible by 1 + 2 + + m ? 1+2+\cdots+m?

No, there are only finitely many such numbers Yes, there are infinitely many such numbers

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2 solutions

1 × 2 × × m 1 + 2 + + m = m ! m ( m + 1 ) 2 = 2 ( m 1 ) ! m + 1 \dfrac{1 \times 2 \times \cdots \times m}{1 + 2 + \cdots + m} = \dfrac{m!}{\dfrac{m(m + 1)}{2}} = \dfrac{2(m - 1)!}{m + 1} .

Now for any even positive integer m m such that m + 1 m + 1 is prime, 2 ( m 1 ) ! 2(m - 1)! will not have m + 1 m + 1 as a term in its prime factorization, i.e., will not be divisible by m + 1 m + 1 . So since there are an infinite number of primes, there will be infinitely many such integers m m .

Naren Bhandari
Apr 8, 2018

1 × 2 × 3 × × k 1 + 2 + + k = 2 ( k 1 ) ! k + 1 \dfrac{1\times 2\times 3\times \cdots \times k}{1+ 2+\cdots +k} = \dfrac{2(k-1)!}{k+1} .Note that k k is an even number so k 1 k-1 and k + 1 k+1 are odd integers where k + 1 > k 1 k+1 > k-1 which shows that ( k 1 ) (k-1) has no factor of k + 1 k+1 .

Further note that 2 2 and k + 1 k+1 are co-prime to each and difference between k 1 k-1 and k + 1 k+1 is of 2 2 so easily we concluded that if k 1 k-1 is prime then k + 1 k+1 will be an odd number and vice or both will be prime. Hence there are infintely many even numbers. m m

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