Browse · MATH
Printjmc
number theory senior
Problem
How many integers less than can be written as the sum of consecutive positive odd integers from exactly 5 values of ?
Solution
Let the first odd integer be , . Then the final odd integer is . The odd integers form an arithmetic sequence with sum . Thus, is a factor of . Since , it follows that and . Since there are exactly values of that satisfy the equation, there must be either or factors of . This means or . Unfortunately, we cannot simply observe prime factorizations of because the factor does not cover all integers for any given value of . Instead we do some casework: If is odd, then must also be odd. For every odd value of , is also odd, making this case valid for all odd . Looking at the forms above and the bound of , must be Those give possibilities for odd . If is even, then must also be even. Substituting , we get Now we can just look at all the prime factorizations since cover the integers for any . Note that our upper bound is now : Those give possibilities for even . The total number of integers is .
Final answer
15