A positive integer is an anchor if every digit of (when written in base 10) is an odd number. Show that there exists an anchor such that the product is not an anchor for any anchor .
Solutions — 4
Solution 1
Solution 1. One such integer is . We show there is no anchor such that is also an anchor for this value of . Let be an anchor. We consider separately the cases and .
For small anchors , we have , , and , none of which are anchors.
When , let the last two digits of be and , with . Then we can write (for some integer ): so that:
The penultimate digit is then which is even, and so cannot be an anchor.
Solution 2
Solution 2. One such integer is . We show there is no anchor such that is also an anchor for this value of . Let be an anchor. We consider separately the cases and .
For small anchors , we have , , and , none of which are anchors.
When , let the last two digits of be and , with . Then we can write (for some integer ): so that
This shows that the second last digit of is equal to , and this is even since is odd. Hence, is not an anchor for any anchor .
Solution 3
Solution 3. One such integer is . We show there is no anchor such that is also an anchor for this value of . The argument below applies to any .
First note that , for any number , has two digits, the sum of which is divisible by 9 and smaller than 18, hence equal to 9. The units digit of is odd if is odd, hence the leading digit is equal to the even number . In particular, the leading digit of is even for , because there are no carries from the multiplications by 1.
If is an anchor, we have with being odd digits. Hence which means that the units digit of is and the hundreds digit is equal to or , both of which are even.
Solution 4
Solution 4. It suffices to find a single anchor such that is a non-anchor for all anchors . There are many such . Here are some cases where with manual checks for :
| m \ n | 91 | 551 | 911 | 931 | 951 | 971 | 991 |
|---|---|---|---|---|---|---|---|
| 3 | 273 | 1653 | 2733 | 2793 | 2853 | 2913 | 2973 |
| 5 | 455 | 2755 | 4555 | 4655 | 4755 | 4855 | 4955 |
| 7 | 637 | 3857 | 6377 | 6517 | 6657 | 6797 | 6937 |
| 9 | 819 | 4959 | 8199 | 8379 | 8559 | 8739 | 8919 |
Here are some cases with :
| m \ n | 99 | 339 | 779 | 919 | 939 | 959 | 979 | 999 |
|---|---|---|---|---|---|---|---|---|
| 3 | 297 | 1017 | 2337 | 2757 | 2817 | 2877 | 2937 | 2997 |
| 5 | 495 | 1695 | 3895 | 4595 | 4695 | 4795 | 4895 | 6895 |
| 7 | 693 | 2373 | 5453 | 6433 | 6573 | 6713 | 6853 | 6993 |
| 9 | 891 | 3051 | 7011 | 8271 | 8451 | 8631 | 8811 | 8991 |
It remains to check that the chosen value of produces non-anchors when multiplied by anchors . To prove this, we suppose more generally that either or . Since is an anchor, has at least two digits and the last two digits are odd. Hence . Consider the multiplication table (mod 20):
| m \ n | 11 | 19 |
|---|---|---|
| 11 | 01 | 09 |
| 13 | 03 | 07 |
| 15 | 05 | 05 |
| 17 | 07 | 03 |
| 19 | 09 | 01 |
In each case, the penultimate decimal digit is even so is not an anchor. According to the remark above, we have tabulated all the solutions for .