function mean( k ) {
	if ( isnan( k ) || k < 0.0 ) {
		return NaN;
	}
	return k;
}
Returns the expected value of a chi-squared distribution.

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

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

@example
var v = mean( 1.0 );
// returns 1.0

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

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