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a(1)=5 because 5^6-6^5=7849=47*167 is a semiprime.
a(1) = 5 because 5^6-6^5 = 47*167
a(2) = 7 because 7^8-8^7 = 23*159463
a(3) = 8 because 8^9-9^8 = 257*354751
a(4) = 11 because 11^12-12^11 = 33479*71549927
a(5) = 17 because 17^18-18^17 = 443881*26757560905578361
a(6) = 18 because 18^19-19^18 = 100417*6015993258685545623
a(7) = 21 because 21^22-22^21 = 10745792197529*9973660056412561
a(8) = 23 because 23^24-24^23 = 92798617729*4576344458074395243073
a(9) = 25 because 25^26-26^25 = 1627*1219220786258356172077730898121187
a(10) = 27 because 27^28-28^27 = 12298336501553*877252504725615101634783073
a(11) = 32 because 32^33-33^32 = 3506869732968391733353*12220478717670771804763962407
a(12) = 47 because 47^48-48^47 = 11*15621013371424880252957237277868559270462038147831682437840584991339231377934499
a(13) = 51 because 51^52-52^51 = 10562756058978342869988055703171*5575962824795589360993690554534422732411612977322491058843
a(14) = 56 because 56^57-57^56 = 5*843980334169667457302970806376511482920948635540290643213973523914715036518308339240201775858865907
a(15) = 59 because 59^60-60^59 = 1994803969065168661575061125592557043358338451845483*8529249434913526091880095870250840825853220069057672947
a(16) = 165 because 165^166-166^165 = 7633959407*16307690786821361595026621717879347561301150483781862339651556401266189322630373265190696672506475741217560239791446654891805648807872536646884416611251422684856600732984767987061831649144878649678190762809385448362714901584206533854093359279076584767352259587745683931159999248465944943129517543272252180930134912057221968601458271001580745436226192252814407
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