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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.

@vam103 You might appreciate the base 16 equation 0x12 = 3+4+5+6

㈰㈪㈫㈬㈭金正月@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