In my opinion, Mathematics is a concept that can be applied to almost anything, but in its essence it remains a highly abstract and scientific subject, so the classification is an adequate one. Art also has a highly emotional component, which can't be measured in mathematical terms.

I have heard it referred to as the Queen Of Sciences as well. It straddles the two really. Unlike most other arts though it is extremely formal which makes it more akin to the sciences. But I find it has inherent beauty as well which is more akin to art, when you get the right answer it is often aesthetically pleasing. But it is 6 of one and half a dozen of the other. EDIT oh and you need to define science and art as well lol And defining what is "art" has baffled philosophers for a very long time :)

The term has two senses. Mathematics is king of science in the sense that it rules over them, all sciences ultimately rely and are based on mathematics for their proofs, tests, rules and laws. The 'harder' the science; how firm or strict the data and the method of finding the data, not how 'difficult' it is to do or understand; the more it relys on math. For example, Math is 'harder' than Physics, ( Math is least susceptible to errors due to interpretation, errors of observation or actions of the observed.) Physics harder than Chemistry which is harder than Biology(Botany harder than Zoology) etc. Math is 'King' of the Universe,'nature' itself seems to be inheritantly 'controlled by math, 'natural laws' obey mathematical law. The spiral of a nautilus shell, seed spacing of sun flowers, other curves follow the same formula. There are other seemingly unrelated things in 'nature' that follow the same mathematical rules, the ratio Pi is repeted in several natural ratios, the frequency and ratio of prime numbers is repeated in natural environments. etc. It is 'Queen', 'ruler' of arts in a similar way. Math controls art. Music has been called the most mathematical of arts, timing, intensity,etc. can be expressed in math formulae. Especially the 'Golden Ratio' a mathematical concept that architects, musicians, painters, sculptors, potters, even picture hangers, etc used for centuries ( and still do) without knowing it was a precise mathematical rule. They didn't really calculate or measure it , it just looked or sounded 'right.' Mathematicians and artists have the 'Golden' rectangle, triangle, circle, and spiral, (remember that nautilus spiral?) But the rules don't have to be strictly followed to create 'acceptable' art. The Queen's rules are not as strict as the King's. Also Math must follow the 'rules' of Science( the King's strict rules,) the collecting of 'data,' 'experimenting' with formulae, recording results, etc. and the results must be repeatable and match with verifiable conditions. No monkeying around with data, 'facts,' or rules to get an 'acceptable' result. In order to be successful the mathematician must follow every rule, not get 'creative' and do something because it 'feels' right.( Some branches of math contradict the rules of other branches, but in each branch the rules must be strictly followed.) But,mathematicians sometimes follow the Queen's less strict, 'artistic' or 'creative' rules. No matter how much finagling with elements of the formula, fudging of facts, the results do notexplain existing conditions. The mather gets artistic and 'creates' or discovers a whole new branch of math and physics. Another instance of 'art' in science is usually only appreciated by mathematicians. You may have heard them describe a formula as 'beautiful,' 'awe inspiring' or'elegant.' Most of us don't get it, it's still a bunch of a few numerals and a bunch of strange symbols. You hardly ever see galleries where folks go to gaze upon truly genius inspired aesthetically pleasing formulae, or even places to buy sofa sized sums by 'starving mathematicians.' That's the main reason math is in 'Science' and not 'Art.'