11. Earnings per Share

Basic earnings per common share is determined by dividing earnings attributable to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration of common stock equivalents. Diluted earnings per share is computed by dividing the earnings attributable to common stockholders by the weighted-average number of common share equivalents outstanding for the period. The treasury stock method is used to determine the dilutive effect of the Company’s stock option grants, SAR, warrants, ESPP awards, and the 2023 Notes.

The following common stock equivalents were excluded in the calculation of diluted earnings per share because their inclusion would be anti-dilutive as applied to the earnings from continuing operations applicable to common stockholders for the three and six months ended June 30, 2018 and 2017:

	Three Months ended June 30,		Six Months ended June 30,
	2018	2017	2018	2017
	(unaudited)		(unaudited)
Warrants to purchase common stock	4,362,485	—	2,816,135	—
Convertible notes	66,961	—	61,104	—
Convertible notes hedges	67	—	61	—
Stock options, stock appreciation rights, and ESPP awards	137,565	122,666	184,760	206,448

The following table sets forth the computation of basic and diluted net earnings per share for the three and six months ended June 30, 2018 and 2017, in thousands of dollars, except share and per share amounts:

	Three Months ended June 30,					Six Months ended June 30,
	2018		2017			2018		2017
	(unaudited)					(unaudited)
Numerator, in thousands:
Net earnings used for calculation of basic EPS	$	30,737	$	17,368		$	57,089	$	27,665

Interest expense on convertible debt	—		58			—		147
Changes in fair value of derivative liabilities	—		(23		)	—		(76		)
Loss on extinguishment of debt	—		103			—		204
Loss on extinguishment of outstanding debt, as if converted	—		(258		)	—		(321		)

Total adjustments	—		(120		)	—		(46		)

Net earnings used for calculation of diluted EPS	$	30,737	$	17,248		$	57,089	$	27,619


Denominator:
Weighted average shares outstanding, basic	51,919,894		50,530,968			51,729,243		50,345,830

Effect of dilutive potential common shares:	—		—			—		—
Shares underlying Convertible Senior Notes	—		421,708			—		551,235
Shares issuable to settle interest make-whole derivatives	—		4,631			—		7,013
Stock options and stock appreciation rights	2,283,414		2,266,407			2,292,698		2,122,245

Total dilutive potential common shares	2,283,414		2,692,746			2,292,698		2,680,493

Weighted average shares outstanding, diluted	54,203,308		53,223,714			54,021,941		53,026,323


Net earnings per share, basic	$	0.59	$	0.34		$	1.10	$	0.55
Net earnings per share, diluted	$	0.57	$	0.32		$	1.06	$	0.52