run PCA, retrieving the first (components) principle components
	return [s0, eig, w0]
	s0: factors
	w0: weights

def PCA(Y, components):
	"""
	run PCA, retrieving the first (components) principle components
	return [s0, eig, w0]
	s0: factors
	w0: weights
	"""

	N,D = Y.shape
	sv = linalg.svd(Y, full_matrices=0);
	[s0, w0] = [sv[0][:, 0:components], np.dot(np.diag(sv[1]), sv[2]).T[:, 0:components]]
	v = s0.std(axis=0)
	s0 /= v;
	w0 *= v;
	return [s0, w0]

	if N>D:
		sv = linalg.svd(Y, full_matrices=0);
		[s0, w0] = [sv[0][:, 0:components], np.dot(np.diag(sv[1]), sv[2]).T[:, 0:components]]
		v = s0.std(axis=0)
		s0 /= v;
		w0 *= v;
		return [s0, w0]
	else:
		K=np.cov(Y)
		sv = linalg.eigh(K)
		std_var = np.sqrt(sv[0])
		pc = sv[1]*std_var[np.newaxis(),0]
		#import ipdb
		#ipdb.set_trace()
		return [pc,std_var]