There Can Be Only One 3

Let's recall the Fibonacci-like sequence from my previous two blog posts titled There Can Be Only One 1 and There Can Only Be One 2.

We start with a digit \(d\) between 0 and 9 and any integer \(s\) that does not contain \(d\). For example, 1 and 2 would be suitable. After that we begin the Fibonacci process using \(d\) and \(s\) as our seeds except that whenever a number appears containing one or more digits \(d\), they are all removed.

So far I've looked at:

• \(d=1\) and \(s=2\) leading to a sequence that loops
• \(d=1\) and \(s=24\) leading to a sequence that loops
• \(d=1\) and \(s=70\) leading to a sequence that loops
• \(d=2\) and \(s=1\) leading to sequence that is probably unbounded
• \(d=2\) and \(s=5\) leading to a sequence that loops

Now let's look at what happens when we set \(d=3\) and \(s=1\). We very quickly end up with a sequence that loops. Here is the trajectory and Figure 1 shows the graph:

3, 1, 4, 5, 9, 14, 2, 16, 18, 4, 22, 26, 48, 74, 122, 196, 18, 214, 22

Figure 1: permalink

Once we set \(s=2\) and maintain \(d=3\), we get a longer sequence but it eventually loops. Here is the trajectory and Figure 2 shows the graph:

3, 2, 5, 7, 12, 19, 1, 20, 21, 41, 62, 10, 72, 82, 154, 26, 180, 206, 86, 292, 78, 70, 148, 218, 66, 284, 50, 4, 54, 58, 112, 170, 282, 452, 74, 526, 600, 1126, 1726, 2852, 4578, 740, 518, 1258, 1776, 4, 1780, 1784, 564, 248, 812, 1060, 1872, 292, 2164, 2456, 4620, 7076, 11696, 18772, 468, 19240, 19708, 8948, 28656, 7604, 6260, 1864, 8124, 9988, 18112, 28100, 46212, 7412, 5624, 106, 570, 676, 1246, 1922, 168, 2090, 2258, 448, 2706, 154, 2860, 14, 2874, 2888, 5762, 8650, 14412, 2062, 16474, 1856, 180

Figure 2: permalink

The sequence reaches a maximum of 46212 with the previous term being 28100. When these two numbers are added we get 74312 that collapses to 7412 and from there it struggles and eventually enters a loop.

It is only with \(d=3\) and \(s=4\) that we get a seemingly unbounded sequence (I tested to 10 million). Here is the trajectory and Figure 3 shows the graph:

3, 4, 7, 11, 18, 29, 47, 76, 12, 88, 100, 188, 288, 476, 764, 1240, 2004, 244, 2248, 2492, 4740, 722, 5462, 6184, 11646, 1780, 1426, 206, 162, 68, 20, 88, 108, 196, 4, 200, 204, 404, 608, 1012, 1620, 262, 1882, 2144, 4026, 6170, 10196, 1666, 11862, 1528, 190, 1718, 1908, 626, 254, 880, 114, 994, 1108, 2102, 210, 212, 422, 64, 486, 550, 106, 656, 762, 1418, 2180, 598, 2778, 76, 2854, 290, 144, 44, 188, 22, 210, 22, 22, 44, 66, 110, 176, 286, 462, 748, 1210, 1958, 168, 2126, 2294, 4420, 6714, 1114, 7828, 8942, 16770, 25712

Figure 3: permalink

So that will probably do it for now. We've seen that it was only with \(d=1\) that we inevitably end up with sequences that loop for values of \(s<100\). For \(d>1\), the sequences can loop or increase without bound depending on the values of \(s\).