Sunday 21 January 2024

One of a Kind?

Consider all the natural numbers up to one million and ask the following question:

What numbers are comprised of the same set of digits as comprise its prime factors? It turns out that 132 is the first such number:$$ \begin{align} 132 &= 2^2 \times 3 \times 11 \\ \{1, 2, 3 \} &= \{1, 2, 3 \} \end{align} $$The second such number is 312:$$ \begin{align} 312 &= 2^3 \times 3 \times 13 \\ \{1, 2, 3 \} &= \{1, 2, 3\} \end{align} $$The third such number is 735:$$ \begin{align} 735 &= 3 \times 5 \times 7^2 \\  \{3, 5, 7\} &= \{3, 5, 7\} \end{align} $$It can be noted that 735 is unique (at least up to one million) in that its factors are single digits and thus none need to be broken down further.

These types of numbers comprise OEIS A035141:


 A035141

Composite numbers \(k\) such that digits in \(k\) and in juxtaposition of prime factors of \(k \) are the same (apart from multiplicity).


There are 96 such numbers in the range up to 40,000:

132, 312, 735, 1255, 1377, 1775, 1972, 3792, 4371, 4773, 5192, 6769, 7112, 7236, 7371, 7539, 9321, 11009, 11099, 11132, 11163, 11232, 11255, 11375, 11913, 12176, 12326, 12595, 12955, 13092, 13175, 13312, 13377, 13491, 13755, 14842, 15033, 15303, 15317, 15532, 16332, 17272, 17276, 17343, 17482, 17973, 17975, 19075, 19276, 20530, 21345, 21372, 22413, 22714, 23535, 24338, 25030, 25105, 27232, 27393, 27944, 31007, 31317, 31419, 31479, 31503, 31592, 31722, 31977, 32024, 32104, 32145, 32612, 32973, 33011, 33327, 33781, 33925, 34112, 34997, 35213, 35262, 35722, 36882, 37115, 37127, 37317, 37359, 37522, 37662, 37741, 37791, 37921, 38385, 39172, 39795

However, it can be restated that 735 is very special, if not unique, because its prime factors (without multiplicity) of 3, 5 and 7 are the same as the digits of the number.

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