#math trivia for #May8: #129 is another prime of the form 2^x+1. How many others will be encountered this year?
— Burt Kaliski Jr. (@modulomathy) May 8, 2012
Nine day-numbers have the form 2^x+1: 2, 3, 5, 9, 17, 33, 65, 129, and 257. Of these, six are prime: 2, 3, 5, 17, 129 and 257. Thus, five others will be encountered, in addition to 129.
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