For how many positive integers is ?
Solution
We count the positive integers for which the product equals 0 and is less than 0 separately. The product equals 0 exactly when one of the factors equals 0. This occurs exactly when equals one of . These are the even integers from 2 to 2018, inclusive, and there are such integers. The product is less than 0 exactly when none of its factors is 0 and an odd number of its factors are negative. We note further that for every integer we have . When , we have and so all 1009 factors are negative, making the product negative. When , we have and all of the other factors are negative, giving 1008 negative factors and so a positive product. When , we have and and all of the other factors are negative, giving 1007 negative factors and so a negative product. This pattern continues giving a negative value for the product for . There are such values (starting at 1, these occur every 4 integers). When , each factor is positive and so the product is positive. Therefore, there are positive integers for which the product is less than or equal to 0.