Distinct consecutive sums!

1243 + 1244 + 1245 + + 1882 = 1 0 6 1288 + 1289 + 1290 + + 1912 = 1 0 6 \begin{aligned} 1243 + 1244 + 1245 + \cdots + 1882 &=& 10^6 \\ 1288 + 1289 + 1290 + \cdots + 1912 &=& 10^6 \end{aligned}

The above shows 2 ways to express 1 0 6 10^6 as the sum of 2 or more consecutive positive integers.

How many other ways can we express 1 0 6 10^6 as the sum of 2 or more consecutive positive integers?


The answer is 4.

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2 solutions

Mark Hennings
Jan 18, 2017

We want to find 0 m < n 0 \le m < n such that 1 0 6 10^6 is the sum of the integers from m + 1 m+1 to n n , so that 1 0 6 = 1 2 n ( n + 1 ) 1 2 m ( m + 1 ) 10^6 \,=\, \tfrac12n(n+1) - \tfrac12m(m+1) , and hence ( n m ) ( n + m + 1 ) = 2000000 = 2 7 × 5 6 (n-m)(n+m+1) \; = \; 2000000 \; = \; 2^7 \times 5^6 Thus n m n-m and n + m + 1 n+m+1 are positive numbers of opposite parity which multiply to 2000000 2000000 , and n + m + 1 > n m n+m+1 > n-m . Thus there are 7 7 options: n + m + 1 n m n m 2 7 × 5 6 1 1000000 999999 2 7 × 5 5 5 200002 199997 2 7 × 5 4 5 2 40012 39987 2 7 × 5 3 5 3 8062 7937 2 7 × 5 2 5 4 1912 1287 5 5 2 7 × 5 1882 1242 5 6 2 7 7876 7748 \begin{array}{|c|c|c|c|} \hline n+m+1 & n-m & n & m \\ \hline 2^7\times5^6 & 1 & 1000000 & 999999 \\ 2^7 \times5^5 & 5 & 200002 & 199997 \\ 2^7 \times5^4 & 5^2 & 40012 & 39987 \\ 2^7 \times5^3 & 5^3 & 8062 & 7937 \\ 2^7 \times5^2 & 5^4 & 1912 & 1287 \\ 5^5 & 2^7 \times 5 & 1882 & 1242 \\ 5^6 & 2^7 & 7876 & 7748 \\ \hline \end{array} The first option is not valid, since it just writes 1000000 1000000 as the sum of the single integer 1000000 1000000 . The other 6 6 are valid solutions, writing 1000000 1000000 as the sum of n m = 5 , 25 , 125 , 625 , 640 , 128 n-m = 5,25,125,625,640,128 consecutive integers respectively. These solutions include the two given in the question, so there are 4 \boxed{4} other solutions.

More generally, to find the number of solutions to "odd * even = 2 k N 2^k N " where N N is odd, then there are 2 possibilities for where to place the powers of 2, and ϕ ( N ) / 2 \phi(N) / 2 ways to pair up the odd divisors. So it's a total of ϕ ( N ) \phi(N) .

Calvin Lin Staff - 4 years, 4 months ago

Oh I don't know, I think one of the moderators/staffs added it. I'll remove it.

Great solution as usual!

Pi Han Goh - 4 years, 4 months ago
Ashish Gupta
Feb 8, 2017

Correcttt!!

Pi Han Goh - 4 years, 4 months ago

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