# What is so unique about these four numbers?

These four numbers look like four ordinary numbers. But I found something unique about these four that I may be only true for one other set of numbers-- unless you raise the bar-may be.

So what is that unique property shared by these four?

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1644 1646 1664 1666

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I edited to include @Nuclear Hoagie's correct comment

• Why limit to the 1600s? As far as I can tell, 1444, 1446, 1464, and 1466 also have this property. Nov 3, 2020 at 14:47
• He says 'I do not think its true' which means he is not sure... Nov 3, 2020 at 14:53
• He is not sure in the question that there are other groups he is just assuming its the case. Nov 3, 2020 at 14:54

They are 4 digit numbers that contain all Roman numerals exactly once

E.g. (1000(M)+500(D)+100(C)+50(L)+10(X)+(-1(I)+5(V)) = 1664)

1644 MDCXLIV, 1646 MDCXLVI, 1664 MDCLXIV, 1666 MDCLXVI

Raising the bar implies putting a bar on top of the letters to multiply them.

• rot13 Ubj qvq lbh svther vg bhg? Jung nobhg gur pbzzrag ba envfvat gur one?
– DrD
Nov 3, 2020 at 14:18
• I think you mean: that contain every Roman numeral exactly once. And raising the bar is putting a bar on top to multiply by 1000. (And it's numbers not necessarily years.) Nov 3, 2020 at 14:34
• Yes that was what I meant Nov 3, 2020 at 14:35