Working with GLOBALs is fragile in all programming languages: It is very hard to find out, where and why the value was set the last time.
Therefore I'd use Paulo's approach ever: "function [y,a]=y(x)" and forward of [a] to all functions, which need it.
Another approach is using a dedicated function to store the value persistently:
function a = Store(Command, a)
case 'get', a = a_;
case 'set', a_ = a;
otherwise error('unknown command');
Although this has the same drawback as GLOBAL ([a] can be influenced from anywhere), you can at least use the debugger to track changes and usage of [a].
But it is at least possible to use GLOBALs (I avoid using "y" as variable and function name at the same time):
function yValue = y(x)
a = 2;
yValue = a * x .^ 2;
Then from the command line or inside another function:
k = y(1:10);