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

Do yoy mean that the name is "Composite number" en.wikipedia.org/wiki/Composit ? Further, multiplicand $$\times$$ multiplier = product en.wikipedia.org/wiki/Multipli , Moreover "divisor" etc en.wikipedia.org/wiki/56_(numb Thanks

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

@j Thank you for telling me! Numbers that are consecutive in order on the formula are beautiful. I search example of additon etc on OEIS too!

I did not find it at all, but a few memorandum of information that I could know for now.

@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

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