by Carl Pierer
Of the many properties a space can have, one of the most intuitively clear seems to be connectedness. Connectedness appears to be the simple property of hanging together in one piece. But how do we make this notion precise? On the one hand, we could think of it as the property that we can reach any point in the space without stepping outside. That is, in other words, that there is a path from any given point to any other point in the space. So, this would make the letter "o" connected, while the letter "i" wouldn't be. This notion corresponds to the topological notion of path connectedness. We say that a space is path connected if for any two points in the space there exists a continuous path from one to the other (which lies entirely inside the space).
On the other hand, we can think of connectedness in slightly different terms. The notion is perhaps clarest if we think about a musical melody. Somehow a succession of sounds turns into a melodic whole, which hangs together in a meaningful way. Breaking up the melody, interrupting it at any one point, can – provided the melody is complex enough – create two full, individual melodies, or one full melody and a somewhat incomplete, unresolved one, or two incomplete succession of notes. If the melody doesn't break into two simpler, shorter melodies, then at least one of the pieces will carry a certain tension that points beyond itself to a resolution of the harmonic build-up. If this is the case, we can think of the melody as in some way the simplest meaningful whole – there is no natural way to separate it into meaningful simpler pieces. This is the second notion of connectedness.
With some poetic licence, we can link the idea of an unresolved melody to the notion of openness, whilst a resolved melody would correspond to closedness, in topological terms. This identification has to be taken with a grain of salt, however, for the two notions – to a topologist – are not opposites. A set can be both open and closed or neither. What does justify the idea, however, is that an unresolved melody, much like an open set, points to a resolution that lies beyond itself: the tone that would form a complete whole, or the limit point that would render the set not open. In any case, we get the second notion of connectedness as a space that does not break up into two, non-empty and non-intersecting, open sets. To translate this into the language of melodies would be to say that a melody is connected if the only way to break it up is that there is one full, simpler melody and one unresolved.
