How many positive integers with can be expressed as the sum of four or more consecutive positive integers?
Solution
We consider first the integers that can be expressed as the sum of exactly 4 consecutive positive integers. The smallest such integer is . The next smallest such integer is . We note that when we move from to , we add 4 to the total (this equals the difference between and since the other three terms do not change). Therefore, the positive integers that can be expressed as the sum of exactly 4 consecutive positive integers are those integers in the arithmetic sequence with first term 10 and common difference 4. Since , these integers are . There are 23 such integers. Next, we consider the positive integers that can be expressed as the sum of exactly 5 consecutive positive integers. The smallest such integer is and the next is . Using an argument similar to that from above, these integers form an arithmetic sequence with first term 15 and common difference 5. Since , these integers are . When we exclude the integers already listed above (30, 50, 70, 90), we obtain . There are 14 such integers. Next, we consider the positive integers that can be expressed as the sum of exactly 6 consecutive positive integers. These integers form an arithmetic sequence with first term 21 and common difference 6. Since , these integers are . When we exclude the integers already listed above , we obtain . There are 12 such integers. Since and this is the smallest integer that can be expressed as the sum of 14 consecutive positive integers, then no is the sum of 14 or more consecutive positive integers. (Any sum of 15 or more consecutive positive integers will be larger than 105.) Therefore, if an integer can be expressed as the sum of consecutive integers, then . We make a table to enumerate the that come from values of with that we have not yet counted: & Smallest & Possible & New \\ 7 & 28 & & \\ 8 & 36 & & \\ 9 & 45 & & 72 \\ 10 & 55 & & None \\ 11 & 66 & & 88 \\ 12 & 78 & 78,90 & None \\ 13 & 91 & 91 & None. In total, there are such .