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%I #15 Aug 29 2017 09:41:03
%S 10529630094750052867957659797284314695762718513641400204044879414141178131103515625,
%T 94766670852750475811618938175558832261864466622772601836403914727270603179931640625,
%U 179003711610750898755280216553833349827966214731903803468762950040400028228759765625,263240752368751321698941494932107867394067962841035005101121985353529453277587890625,347477793126751744642602773310382384960169710950166206733481020666658878326416015625
%N One powerful arithmetic progression with nontrivial difference and maximal length.
%C This sequence has the maximal length of a powerful arithmetic progression for which the k-th term is a k-th power.
%C The originating sequence is 1, 9, 17, 25, 33 with difference 8. This sequence is multiplied by 3^24*5^30*11^24*17^20 to generate a(n) with common difference 84237040758000422943661278378274517566101748109131201632359035313129425048828125000.
%C Note that this sequence is just an example of a maximal progression. Similar progressions with smaller terms are provided by 2^15*3^24*5^40*13^24 * {11, 18, 25, 32, 39}, 37^24 * {213, 169, 125, 81, 37}, or, if negative terms are allowed, by 2^15*5^20 * {11, 8, 5, 2, -1}. - _Giovanni Resta_, Aug 29 2017
%H John P. Robertson, <a href="http://www.jstor.org/stable/2695599">The maximal length of a powerful arithmetic progression</a>, American Mathematical Monthly 107 (2000), 951.
%e a(1) is obviously a first power.
%e a(2) = 307841957589849138828884412917083740234375^2 is a square.
%e a(3) = 5635779747116948576103515625^3 is a third power.
%e a(4) = 716288998461106640625^4 is a fourth power.
%e a(5) = 51072299355515625^5 is a fifth power.
%Y Cf. A050923.
%K nonn,fini,full
%O 1,1
%A _Martin Renner_, Aug 16 2017
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