Wikipedia:Reference desk/Archives/Mathematics/2007 February 4

= February 4 =

Another triangle question
Rather similar to the angle-bisector questions above, is it possible to determine the sides of a triangle, given the perpendicular distances from the circumcentre to the sides?…81.151.247.173 00:09, 4 February 2007 (UTC)

Rectangular numbers
Please inform me what are rectangular numbers —Preceding unsigned comment added by 168.167.72.20 (talk • contribs)


 * The term rectangular number does not seem to have a standard definition. Looking for references, I found several sources where the term is used to refer to an integer of the form n(n+1) - so the first few rectangular numbers using this definition are 2, 6, 12, 20, 30. These numbers are also called pronic numbers or oblong numbers. But other sources use rectangular number to mean a composite number, or, more rarely, a composite number that is not a square number. Gandalf61 09:46, 4 February 2007 (UTC)

Set of all integers
Can I have the set-builder notation for "x must be in the set of all integers"?

I know that it is usually represented by the letter I (or Z?), but how can I express that x must be a member of that set e.g. for a formula?

86.146.72.220 10:55, 4 February 2007 (UTC)


 * The letter for integer is indeed blackboard bold Z (from the German Zahlen). The set is written as $$\lbrace x : x \in \mathbb{Z} \rbrace$$. For more info, see set-builder notation. Laïka  11:29, 4 February 2007 (UTC)