1 2 4 3 + 1 2 4 4 + 1 2 4 5 + ⋯ + 1 8 8 2 1 2 8 8 + 1 2 8 9 + 1 2 9 0 + ⋯ + 1 9 1 2 = = 1 0 6 1 0 6
The above shows 2 ways to express 1 0 6 as the sum of 2 or more consecutive positive integers.
How many other ways can we express 1 0 6 as the sum of 2 or more consecutive positive integers?
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More generally, to find the number of solutions to "odd * even = 2 k N " where N is odd, then there are 2 possibilities for where to place the powers of 2, and ϕ ( N ) / 2 ways to pair up the odd divisors. So it's a total of ϕ ( N ) .
Oh I don't know, I think one of the moderators/staffs added it. I'll remove it.
Great solution as usual!
Correcttt!!
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We want to find 0 ≤ m < n such that 1 0 6 is the sum of the integers from m + 1 to n , so that 1 0 6 = 2 1 n ( n + 1 ) − 2 1 m ( m + 1 ) , and hence ( n − m ) ( n + m + 1 ) = 2 0 0 0 0 0 0 = 2 7 × 5 6 Thus n − m and n + m + 1 are positive numbers of opposite parity which multiply to 2 0 0 0 0 0 0 , and n + m + 1 > n − m . Thus there are 7 options: n + m + 1 2 7 × 5 6 2 7 × 5 5 2 7 × 5 4 2 7 × 5 3 2 7 × 5 2 5 5 5 6 n − m 1 5 5 2 5 3 5 4 2 7 × 5 2 7 n 1 0 0 0 0 0 0 2 0 0 0 0 2 4 0 0 1 2 8 0 6 2 1 9 1 2 1 8 8 2 7 8 7 6 m 9 9 9 9 9 9 1 9 9 9 9 7 3 9 9 8 7 7 9 3 7 1 2 8 7 1 2 4 2 7 7 4 8 The first option is not valid, since it just writes 1 0 0 0 0 0 0 as the sum of the single integer 1 0 0 0 0 0 0 . The other 6 are valid solutions, writing 1 0 0 0 0 0 0 as the sum of n − m = 5 , 2 5 , 1 2 5 , 6 2 5 , 6 4 0 , 1 2 8 consecutive integers respectively. These solutions include the two given in the question, so there are 4 other solutions.