What is the sum of all numbers q which can be written in the form q=ba where a and b are positive integers with b≤10 and for which there are exactly 19 integers n that satisfy q<n<q?
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Suppose that a number q has the property that there are exactly 19 integers n with q<n<q. Suppose that these 19 integers are m,m+1,m+2,…,m+17,m+18. Then q<m<m+1<m+2<⋯<m+17<m+18<q. This tells us that q−q>(m+18)−m=18 because q−q is as small as possible when q is as small as possible and q is as large as possible. Also, since this is exactly the list of integers that is included strictly between q and q, then we must have m−1≤q<m<m+1<m+2<⋯<m+17<m+18<q≤m+19. In other words, neither m−1 nor m+19 can satisfy q<n<q. This tell us that q−q≤(m+19)−(m−1)=20. Therefore, we have that 18<q−q≤20. Next, we use 18<q−q≤20 to get a restriction on q itself. To have q−q>18, we certainly need q>18. But if q>18, then q>18>4. Furthermore, q−q>18 and q>4 give q−4>q−q>18 and so q>22. Next, note that q−q=q(q−1). When q is larger than 1 and increases, each factor q and q−1 increases, so the product q−q increases. When q=25,q−q=25−5=20. Since we need q−q≤20 and since q−q=20 when q=25 and since q−q is increasing, then for q−q≤20, we must have q≤25. Therefore, 18<q−q≤20 tells us that 22<q≤25. So we limit our search for q to this range. When q=22,q≈4.69, and so the integers n that satisfy q<n<q are 5,6,7,…,20,21, of which there are 17. When 22<q≤23, we have 4<q<5 and 22<q≤23, which means that the integers n that satisfy q<n<q are 5,6,7,…,20,21,22, of which there are 18. When 23<q≤24, we have 4<q<5 and 23<q≤24, which means that the integers n that satisfy q<n<q are 5,6,7,…,20,21,22,23, of which there are 19. When 24<q<25, we have 4<q<5 and 24<q<25, which means that the integers n that satisfy q<n<q are 5,6,7,…,20,21,22,23,24, of which there are 20. When q=25,q=5 and so the integers that satisfy q<n<q are 6,7,…,20,21,22,23,24, of which there are 19. Therefore, the numbers q for which there are exactly 19 integers n that satisfy q<n<q are q=25 and those q that satisfy 23<q≤24. Finally, we must determine the sum of all such q that are of the form q=ba where a and b are positive integers with b≤10. The integers q=24 and q=25 are of this form with a=24 and a=25, respectively, and b=1. The q between 23 and 24 that are of this form with b≤4 are 2321=247,2331=370,2332=371,2341=493,2343=495. Notice that we don't include 2342 since this is the same as the number 2321. We continue by including those satisfying 5≤b≤10 and not including equivalent numbers that have already been included with smaller denominators, we obtain 2321,2331,2332,2341,2343,2351,2352,2353,2354,2361,2365,2371,2372,2373,2374,2375,2376,2381,2383,2385,2387,2391,2392,2394,2395,2397,2398,23101,23103,23107,23109. There are 31 numbers in this list. Each of these 31 numbers equals 23 plus a fraction between 0 and 1. With the exception of the one number with denominator 2, each of the fractions can be paired with another fraction with the same denominator to obtain a sum of 1. Therefore, the sum of all of these q between 23 and 24 is 31(23)+21+15(1)=72821, because there are 31 contributions of 23 plus the fraction 21 plus 15 pairs of fractions with a sum of 1. Finally, the sum of all q of the proper form for which there are exactly 19 integers that satisfy q<n<q is 72821+25+24=77721.
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