Today I turned 77 years of age and my equivalent diurnal age is 28124 which has the following factorisation:$$28124=2 \times 2 \times 79 \times 89$$Though this number is composite, it has numerous prime number associations. Let's examine some of them beginning with its sum of digits, sum of digits squares and sum of digits cubed:$$ \begin{align} 2 + 8 + 1 + 2 + 4 &=17 \text{ (prime)} \\2^2+8^2+1^2+2^2+8^4 &= 89 \text{ (prime)} \\2^3+8^3+1^3+2^3+8^3 &= 593 \text{ (prime)} \end{align}$$The number is only one step removed from its home prime because:$$28124=2 \times 2 \times 79 \times 89 \rightarrow 227989 \text{ (prime)}$$The number is also a member of OEIS A048381: numbers such that replacing each nonzero digit with the n-th prime (replacing each 0 digit with a 1) yields a prime. Thus:$$28124 \rightarrow 319237 \text{ (prime)}$$The number has a binary complement that is prime. The binary complement of a number is determined by changing the number to binary and swapping any 0's for 1's and vice versa. Thus:$$ \begin{align} 28124_{10} &= 110110111011100_2 \\ &\rightarrow 001001000100011_2 \\ &=4643_{10} \text{ (prime)} \end{align}$$The number is quickly captured by the prime 28109 under the ODD(+) and EVEN(-) algorithm where the sum of the odd digits is added to the number and the sum of the even digits is subtracted recursively until a fixed point is reached or a loop is entered. Here is the trajectory is simply:$$ \begin{align} 28124 &\rightarrow 28124 + 1 -(2 + 8 + 2 + 4) \\ &=28124 + 1 - 16 \\ &=28109 \text{ (prime)} \end{align}$$The number can be considered as a concatenation of powers of the prime 2 because:$$ 28124 = 2^1\, | \,2^3 \,| \,2^0 \,| \,2^1 \,| \, 2^2 $$where | represents concatenation. The number can be generated by adding the prime sum (13) of the digits of the prime 28111 to itself. Thus:$$28111+13=28124$$The digits of the number can be rearranged to form the following primes:$$22481, 24281, 24821, 42281, 42821, 48221, 82241, 82421, 84221$$The position 28124 in the Recaman Sequence is reached after a prime number of iterations:$$0 \rightarrow 28124 \text{ requires } 34183 \text{ (prime) iterations}$$
Friday, 3 April 2026
Monday, 30 March 2026
Number Testing for the Recaman Sequence
An interesting exercise is to determine how many iterations are required to reach a given number within the Recaman sequence. This is OEIS A057167. The initial terms that are listed in the OEIS are:
0, 1, 4, 2, 131, 129, 3, 5, 16, 14, 12, 10, 8, 6, 31, 29, 27, 25, 23, 99734, 7, 9, 11, 13, 15, 17, 64, 62, 60, 58, 56, 54, 52, 50, 48, 46, 44, 42, 40, 38, 111, 22, 20, 18, 28, 30, 32, 222, 220, 218, 216, 214, 212, 210, 208, 206, 204, 202, 200, 198, 196
Notice 99734 that I've marked in red in the terms above. This corresponds to 19 and so a remarkable 99734 iterations are required to reach this modest number. Looking further at the \(b\) table, it can be seen that 61 requires an even more impressive 181653 iterations. Interestingly 76 requires the exact same number of iterations. 133 requires only slightly fewer at 181605 and 223 fewer still at 181545. These "spikes" are relatively infrequent. The next significant one is 879 that requires 328002 iterations. However, these are mere blips compared to the staggering number of iterations required for a number like 2406 (see Table 1).
Also of interest is the maximum number reached in the trajectory leading to a given number. I got Gemini to write a SageMath program that will calculate these two statistics (permalink). Let's say we enter the number 61. Here is the output:
Target Number: 61
Iterations Required: 181653
Highest Value Attained: 881467
I've incorporated this into my daily number analysis. My diurnal age today is 28120 and entering this number, the following output is obtained:
Position in Recaman Sequence:
Target Number: 28120
Iterations Required: 34191
Highest Value Attained: 180120
I also got Gemini to create a graph based on the OEIS A057167 b-table data that lists the number of iterations for the first 20,000 numbers. The program also lists the top ten values in that range. See Figure 1.
![]() |
Figure 1: permalink |
Table 1 shows the top ten in terms of number of iterations required:
![]() |
Table 1: permalink |
Wednesday, 2 July 2025
Prime Factor Sequences
I'm surprised I've not come across this type of sequence before. It has two variants and they are generated iteratively as follows:
- number --> sum of prime factors without multiplicity
For example, 24 with factors of 2 and 3 gives 5 and terminates after just one step - number --> sum of prime factors with multiplicity
For example, 24 with factors of 2, 2, 2 and 3 gives 11 and terminates after just one step
![]() |
Figure 1: permalink |
If the sum of prime factors with multiplicity is considered then we get OEIS A121360 with sequence members 1, 8, 14, 26, 62, 134, 393, 1257, 4659, 9314, 27933 up to 100,000. The trajectories are shown in Figure 2 with length indicating the number of steps or iterations:![]() |
| Figure 2: permalink |
![]() |
Figure 3: permalink |
Tuesday, 12 March 2024
27372: Another Palindromic Day
Days like today, when I turn 27372 days old, pop up every one hundred days during the course of a millennium of days and there is a 110 day gap between millennia. So, for example, from 27972 to 28082, there will be a gap of 110 days. Today's number shares some important properties with another palindrome, 26362, that I created a post about on June 6th 2021. It was titled 26362: Another Special Palindrome.
One property that the two share is that they are both members of OEIS A070001:
A070001 | Palindromic integers > 0, whose 'Reverse and Add!' trajectory (presumably) does not lead to another palindrome. |
Up to 40000, the members of this sequence are not numerous and they are:
4994, 8778, 9999, 11811, 19591, 22822, 23532, 23632, 23932, 24542, 24742, 24842, 24942, 26362, 27372, 29792, 29892, 33933, 34543, 34743, 34943, 39493
It can be seen that 26362 and 27372 are consecutive and 1010 days apart in terms of my diurnal age. As I wrote in the post previously alluded to:
These palindromes are not regarded as potential Lychrel numbers because they are already palindromes and some of them are the result or end point of \(k\) + reverse(\(k\)) iterations. However, some are not and these, I think, deserve special consideration. These are:
19591, 23532, 23932, 24542, 24742, 24942, 26362, 27372, 29792, 33933, 34543, 34743, 34943, 39493
So 26362 and 27372 are paired again and they are only the 7th and 8th palindromes to have the simultaneous property that:
- they cannot be derived from \(k\) + reverse(\(k\)) for one or more values of \(k\)
- their Reverse and Add trajectories (presumably) do not lead to another palindrome
A045960 | Palindromic even lucky numbers. |
Saturday, 24 February 2024
Thinning the Ranks
Every now and again I encounter a number associated with my diurnal age that seems to have no interesting properties from my perspective. In such cases, I have to be a little creative and such is the case for the number: 27355. After a little thought, I experimented with home primes and asked the question: how many iterations of factorise and concatenate are required to reach a home prime. Well, it turns out that three iterations are required:$$ \begin{align} 27355 &= 5 \times 5471 \rightarrow 55471 \\ 55471 &=13 \times 17 \times 251 \rightarrow 1317251 \\1317251 &=13 \times 19 \times 5333 \rightarrow 13195333 \end{align}$$Now in the range up to 40,000, there are well over 3,000 composite numbers with this property so it's hardly very special. These numbers belong to OEIS A046423:
A046423 | Numbers requiring 3 steps to reach a prime under the prime factor concatenation procedure. |
However, I noticed that the home prime for 27355 had exactly half of its digits equal to 3. It occurred to me to investigate how many composite numbers in the range up to 40,000 and belonging to OEIS A046423 had at least half their digits equal to 3. It turned out that there were only 191 numbers (permalink). This is what I meant by "thinning the ranks". It's interesting to investigate the frequency for other digits that comprise at least half of the digits of the home prime. Here are the statistics:
- 0 --> no numbers
- 1 --> 120 numbers
- 2 --> 2 numbers
- 3 --> 191 numbers
- 4 --> 3 numbers
- 5 --> 2 numbers
- 6 --> no numbers
- 7 --> 69 numbers
- 8 --> no numbers
- 9 --> 11 numbers
- 1 iteration --> 600 numbers
- 2 iterations --> 441 numbers
- 3 iterations --> 191 numbers
- 4 iterations --> 105 numbers
- 5 iterations --> 48 numbers
- 6 iterations --> 34 numbers
- 7 iterations --> 12 numbers
- 8 iterations --> 6 numbers
- 9 iterations --> no numbers
- 4017 --> 3337715393
- 4242 --> 23393307373
- 4667 --> 33433193
- 7474 --> 23393307373
- 31355 --> 3332943503
- 39309 --> 3337715393
- digit 0 occurs 3 times --> no numbers
- digit 1 occurs 3 times --> 440 numbers
- digit 2 occurs 3 times --> 645 numbers
- digit 3 occurs 3 times --> 565 numbers
- digit 4 occurs 3 times --> 4 numbers
- digit 5 occurs 3 times --> 83 numbers
- digit 6 occurs 3 times --> 1 number
- digit 7 occurs 3 times --> 139 numbers
- digit 8 occurs 3 times --> 1 number
- digit 9 occurs 3 times --> 23 numbers
A187073 | Composite square-free numbers whose average prime factor is a prime number. |
- 15369 = 3 * 47 * 109 with prime average of 53
- 15515 = 5 * 29 * 107 with prime average of 47
- 17135 = 5 * 23 * 149 with prime average of 59
- 22865 = 5 * 17 * 269 with prime average of 97
- 24215 = 5 * 29 * 167 with prime average of 67
- 26619 = 3 * 19 * 467 with prime average of 163
- 29949 = 3 * 67 * 149 with prime average of 73
- 32809 = 7 * 43 * 109 with prime average of 53
- 33065 = 5 * 17 * 389 with prime average of 137
- 33909 = 3 * 89 * 127 with prime average of 73
- 36879 = 3 * 19 * 647 with prime average of 223
- 37639 = 7 * 19 * 283 with prime average of 103
- 39759 = 3 * 29 * 457 with prime average of 163
Wednesday, 18 January 2023
What's Special About 516493?
5 1 6 4 9 3
I got to thinking about primes that remain prime under the operation of prime + digit sum. The acronym "sod" standing for sum of digits is commonly used in this context. There are many primes that satisfy. For example, 11 has a digit sum of 2 and 11 + 2 = 13 which is prime. If we repeat the operation we get 13 + 4 = 17 which is still prime. However, repeating the operation again yields 17 + 8 = 25 which is composite.
The primes that remain prime for only one application of the prime + sod operation constitute OEIS A048523 (permalink):
A048523 | Primes for which only one iteration of 'Prime plus its digit sum equals a prime' is possible. |
A048524 | Primes for which only two iterations of 'Prime plus its digit sum equals a prime' are possible. |
A048525 | Primes for which only three iterations of 'Prime plus its digit sum equals a prime' are possible. |
A048526 | Primes for which only four iterations of 'Prime plus its digit sum equals a prime' are possible. |
A048527 | Primes for which only five iterations of 'Prime plus its digit sum equals a prime' are possible. |
Saturday, 21 August 2021
Analysis of a Recursive Process
Today I turned 26438 days old and one of the properties of this number is that it's a member of OEIS A048128:
A048128 | Becomes prime or 4 after exactly 6 iterations of f(x) = sum of prime factors of x. |
- 0: 09.59%
- 1: 12.29%
- 2: 15.21%
- 3: 15.24%
- 4: 13.44%
- 5: 15.31%
- 6: 12.63%
- 7: 04.89%
- 8: 01.18%
- 9: 00.21%
- 10: 0.020%
- 11: 0.020%
- 27933, 9314, 4659, 1556, 393, 134, 69, 26, 15, 8, 6, 5
- 55694, 27849, 9286, 4645, 934, 469, 74, 39, 16, 8, 6, 5
How do things change when we consider only the prime factor without multiplicity? Here are the results:- 0: 09.59%
- 1: 11.81%
- 2: 14.78%
- 3: 26.51%
- 4: 21.27%
- 5: 11.08%
- 6: 3.955%
- 7: 0.899%
- 8: 0.093%
- 9: 0.006%
- 10: 0.000%
- 11: 0.000%
Sunday, 6 June 2021
26362: Another Special Palindrome
The day that I turned 26262 days old I dedicated a post to the number and titled it 26262: A Special Palindrome. That was on February 21st 2021. Today I'm enjoying the next successive palindromic day, having turned 26362 days old.
Numbers \(n\) that are of the form \(k\) + reverse(\(k\)) for at least one \(k\). |
0, 2, 4, 6, 8, 10, 11, 12, 14, 16, 18, 22, 33, 44, 55, 66, 77, 88, 99, 101, 110, 121, 132, 141, 143, 154, 161, 165, 176, 181, 187, 198, 201, 202, 221, 222, 241, 242, 261, 262, 281, 282, 302, 303, 322, 323, 342, 343, 362, 363, 382, 383, 403, 404, 423, 424, 443
131 requires 1 steps to reach the palindrome 262151 requires 2 steps to reach the palindrome 505171 requires 2 steps to reach the palindrome 585191 requires 4 steps to reach the palindrome 2552212 requires 1 steps to reach the palindrome 424232 requires 1 steps to reach the palindrome 464252 requires 2 steps to reach the palindrome 909272 requires 2 steps to reach the palindrome 989292 requires 8 steps to reach the palindrome 233332313 requires 1 steps to reach the palindrome 626333 requires 1 steps to reach the palindrome 666353 requires 3 steps to reach the palindrome 4444373 requires 4 steps to reach the palindrome 9559393 requires 4 steps to reach the palindrome 9339434 requires 1 steps to reach the palindrome 868
26362 as it turns out is a member of OEIS A070001:
A070001 | Palindromes whose 'Reverse and Add' trajectory (presumably) does not lead to another palindrome. |
The initial members of this sequence are:
4994, 8778, 9999, 11811, 19591, 22822, 23532, 23632, 23932, 24542, 24742, 24842, 24942, 26362, 27372, 29792, 29892, 33933, 34543, 34743, 34943, 39493, 44744, 46064, 46164, 46364, 46564, 46964, 47274, 47574, 48284, 48584, 48684, 48884
These palindromes are not regarded as potential Lychrel numbers because they are already palindromes and some of them are the result or end point of \(k\) + reverse(\(k)\) iterations. However, some are not and these, I think, deserve special consideration:
19591, 23532, 23932, 24542, 24742, 24942, 26362, 27372, 29792, 33933, 34543, 34743, 34943, 39493, 44744, 46164, 46364, 46564, 46964, 47574, 48584
So 26362 is only the 7th palindrome to have the simultaneous property that:
- it cannot be derived from \(k\) + reverse(\(k\)) for one or more values of \(k\)
- its Reverse and Add trajectory (presumably) does not lead to another palindrome
- Palindromic Cyclops Numbers on August 10th 2020
- L-th Order Palindromes on January 24th 2019
- Lychrel Numbers on September 14th 2016
- 22, Reverse and Add on January 7th 2016
- More on AntiDivisors on February 28th 2021
(mention made of palindromes whose sum of anti-divisors is palindromic). - Remembering Reverse and Add, Palindromes and Trajectories on June 22nd 2016
A320516 | Palindromic wing primes that are also Lychrel candidates. |
7774777, 777767777, 77777677777, 99999199999, 1111118111111, 7777774777777, 111111181111111, 333333373333333, 77777777677777777, 99999999299999999, 9999999992999999999, 33333333333733333333333, 77777777777677777777777, 333333333333373333333333333
More information about PWPs can be found here. Palindromes and any numbers can be tested for Lychrel candidature using SageMathCell. A permalink is attached to the screenshoot in Figure 1.
![]() |
| Figure 1: permalink |
Sunday, 25 August 2019
The PrimeLatz Conjecture
- The 3x+1 Problem on the 1st November 2016
- The Collatz Conjecture Revisited on the 15th March 2018
- The 17x+1 Map on the 18th March 2018
The so-called Collatz Trajectory refers to the sequence of numbers generated by the following rule. Start with any positive integer. If the number is even, divide it by 2. If the number is odd, multiply by 3 and add 1. The conjecture is that the sequence of numbers thus generates will always lead to 1. So far, no exceptions have been found.Most small initial values have a very small orbit of few more than the 30 elements of the loop.
N = 83 = a(0) is the most remarkable exception (having an orbit of 16180 + 30 elements), which motivates this sequence.
N = 443 = A293978(0) is another exception, with an orbit of 9066+30 elements, and N = 209 also has a comparatively large orbit of 941 + 30 elements, distinct from those of 83 and 443.
The initial value a(0) = 83 is odd, so we add to 83 the next 3 primes (89, 97 and 101) to get a(1) = 370.
370 is even, so we divide by 2 to get a(2) = 185, and so on.
After 8337 iterations, we get a(8337) = 10780054699424618132644155893087038044817868609971935265882538442720.
This is the largest value we will reach. Since this is even we divide by 2 to get a(8338). The result a(8338) is again even, so we divide by 2 once more to get a(8339), and so on...
After iteration 16171, we reach a(16171) = 768. The next 8 iterations consist in dividing by 2, until we get a(16179) = 3. Since this is odd, we add the next three primes (5, 7 and 11) to reach a(16180) = 26 = A193230(14). This is an element of the loop: 30 iterations later, we get again 26, and the sequence has become periodic.All numbers less than 100 million eventually fall into the following loop:
30,15,74,37,168,84,42,21,104,52,26,13,72,36,18
ADDENDUM 10th May 2020:
I'm now approaching my 26000th day on Earth and I've noticed that, in the orbit of 83, there is a cluster of numbers between 25000 and 31000 with wide gaps on either side. Figure 1 shows what I mean:
![]() |
| Figure 1 |
There is a gap of over 10000 from 15456 to 25710 and then a gap of over 20000 from 30912 to 51536. As I write this on the 10th May 2020, I'm 25970 days old and this number is in the 83 orbit. There are a total of 54 numbers that fall in the range from 25710 to 30912.
The full orbit of 83 can be viewed, in comma-delimited format, by following this link (it's 176 pages long):https://docs.google.com/document/d/1uP-sBqDNjbhkEqK8IEDG-doN2QeYXnq0K8QObWkzPWU/edit?usp=sharing













