My Einstein number is 5 via the following path:

Valerio Cappellini coauthored with Fabio Benatti − MR2027615 (2005e:81072)
Fabio Benatti coauthored with Heide Narnhofer − MR0952455 (89i:82016)
Heide Narnhofer coauthored with John R. Klauder − MR0421449 (54 #9453)
John R. Klauder coauthored with Valentine Bargmann − MR0290680 (44 #7860)
Valentine Bargmann coauthored with Albert Einstein − MR0004790 (3,62b)