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Do numbers like \(12=3\times 4\) and \(56=7×8\) have a name? Are there any more?

@unknown Notice that the first has digits 1,2,3,4 in order. The second has digits 5,6,7,8 in order.

@vam103 You're looking for integer solutions to \(f(a) = 0\) where \(f(a) = a\cdot10^{\lceil \log_{10} (a+1)\rceil} + (a+1) - (a+2)(a+3)\). That odd operating with the ceil \(\lceil\cdot\rceil\) and \(\log\) is concatentation.

\(a=1,\) and \(a=5\) are two. \(f\) is easier to differentiate than it looks at first glance, just remember that floor and ceiling functions are locally constant. Use that to see that \(f\) is increasing before \(a=1,\) and decreasing after \(a=5\). They're the only two.

I have poor English, and PPAP.@unknown@mathstodon.xyzDo yoy mean that the name is "Composite number" https://en.wikipedia.org/wiki/Composite_number ? Further, multiplicand \(\times\) multiplier = product https://en.wikipedia.org/wiki/Multiplication , Moreover "divisor" etc https://en.wikipedia.org/wiki/56_(number) Thanks