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Saturday, 22 April 2023

Concatenating Factors of Sphenic Numbers

The number associated with my diurnal age today, 27047, is a sphenic number and it factorises to 17 x 37 x 43. When these factors are concatenated to form a new number, 173743, then this number is prime. By convention, the concatenation is effected in ascending order but I wondered how the primeness would hold up if all possible orders were considered. The answer is as follows with True meaning is prime and False meaning is not:

173743 True

174337 True

371743 False

374317 True

433717 False

431737 False

So 27047 scores 50% with half the concatenations being prime and half not. The natural follow up was to ask what sphenic numbers hold up 100% of the time. It didn't take too long to construct a SageMath algorithm to determine this in the range up to one million (permalink). There are only 15 such numbers in that range and they are:

3311, 27181, 32153, 41237, 53977, 86507, 110971, 125069, 208579, 256413, 500981, 543337, 853811, 901949, 964481

The factorisations of these numbers are:

3311 = 7 * 11 * 43

27181 = 7 * 11 * 353

32153 = 11 * 37 * 79

41237 = 7 * 43 * 137

53977 = 7 * 11 * 701

86507 = 19 * 29 * 157

110971 = 7 * 83 * 191

125069 = 7 * 17 * 1051

208579 = 7 * 83 * 359

256413 = 3 * 127 * 673

500981 = 13 * 89 * 433

543337 = 17 * 31 * 1031

853811 = 7 * 283 * 431

901949 = 19 * 37 * 1283

964481 = 7 * 211 * 653

Checking with the OEIS, I found that this sequence of numbers is listed as OEIS A180679:


 A180679



Numbers with three distinct prime factors which when concatenated in any order form prime numbers.



A longer list of sequence members is provided:

3311, 27181, 32153, 41237, 53977, 86507, 110971, 125069, 208579, 256413, 500981, 543337, 853811, 901949, 964481, 1053787, 1144171, 1197851, 1215731, 1344539, 1385189, 1433659, 1549603, 1674741, 1681547, 1699481, 1973479, 2028181

One could carry out a similar investigation on semiprimes and numbers with four distinct prime factors, although the OEIS comments note that there is no term with four distinct prime factors under 108. Interestingly, the second number is the sequence, 27181, is coming up very soon in my diurnal age count. It is only 134 days away.

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