function mean( v ) {
	if ( isnan( v ) || v <= 1.0 ) {
		return NaN;
	}
	return 0.0;
}
Returns the expected value of a Student's t distribution.

@param {NonNegativeNumber} v - degrees of freedom
@returns {NonNegativeNumber} expected value

@example
var v = mean( 9.0 );
// returns 0.0

@example
var v = mean( 2.0 );
// returns 0.0

@example
var v = mean( 0.5 );
// returns NaN

@example
var v = mean( -0.2 );
// returns NaN

@example
var v = mean( NaN );
// returns NaN