Friday, 25 July 2025

27872: Another Palindromic Number


For the last 800 days they've been coming around every 100 days and today marks yet another palindromic day as I turn 27872 days old. I've mentioned one of the properties of this number in my post titled Difference of Two Cubic Numbers. I noted that this palindrome is a difference of two cubes:$$27872=38^3-30^3$$Another property of this palindrome is that it is the sum of two prime palindromes in two different ways:$$ \begin{align} 27872 &= 11311 + 16561\\ &=12421 + 15451 \end{align}$$In the range up to 40000, there are only 25 palindromes with this property and these are the initial terms of OEIS A356854:

282, 484, 858, 888, 21912, 22722, 23832, 24642, 25752, 26662, 26762, 26862, 26962, 27672, 27772, 27872, 27972, 28482, 28782, 28882, 28982, 29692, 29792, 29892, 29992

27872 also has the property that it is the smallest palindrome with exactly seven prime factors, counted with multiplicity. This qualifies it for membership in OEIS A076886: smallest palindrome with exactly \(n\) prime factors (counted with multiplicity). See Figure 1 where the initial terms with their \(n\) values are listed.

Figure 1

In 100 days I'll be able to celebrate 27972 that's also a member of OEIS A356854. After that, it is only 31 days to the next palindromic day: 30003.

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