function kurtosis( v ) {
	if ( isnan( v ) || v <= 2.0 ) {
		return NaN;
	}
	if ( v <= 4.0 ) {
		return PINF;
	}
	return 6.0 / ( v - 4.0 );
}
Returns the excess kurtosis of a Student's t distribution.

@param {NonNegativeNumber} v - degrees of freedom
@returns {NonNegativeNumber} excess kurtosis

@example
var v = kurtosis( 9.0 );
// returns 1.2

@example
var v = kurtosis( 5.0 );
// returns 6.0

@example
var v = kurtosis( 3.0 );
// returns Infinity

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

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

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