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A249780 Product of lowest and highest prime factors of 2^n-1.

%I A249780 #20 Jun 02 2025 10:16:07
%S A249780 9,49,15,961,21,16129,51,511,93,2047,39,67092481,381,1057,771,
%T A249780 17179607041,219,274876858369,123,2359,2049,8388607,723,55831,24573,
%U A249780 1838599,381,486737,993,4611686014132420609,196611,4196353,393213,3810551,327,137438953471,1572861,849583,185043
%N A249780 Product of lowest and highest prime factors of 2^n-1.
%H A249780 Chai Wah Wu, <a href="/A249780/b249780.txt">Table of n, a(n) for n = 2..200</a>
%F A249780 a(n) = A005420(n) * A049479(n)
%e A249780 The lowest and higest prime factors of 2^6-1 are 3 and 7, so A(6) = 21
%p A249780 a:= proc(n) local F; F:= numtheory:-factorset(2^n-1); min(F)*max(F) end proc:
%p A249780 seq(a(n),n=2..50); # _Robert Israel_, Nov 05 2014
%t A249780 plhpf[n_]:=Module[{fn=FactorInteger[n]},fn[[1,1]]fn[[-1,1]]]; Table[plhpf [2^n-1],{n,2,40}] (* _Harvey P. Dale_, May 23 2020 *)
%o A249780 (PARI) for(n=2, 50, p=2^n-1; print1(factor(p)[1, 1]*factor(p)[#factor(p)[, 1], 1], ", ")) \\ _Derek Orr_, Nov 05 2014
%o A249780 (Python)
%o A249780 from sympy import primefactors
%o A249780 A249780_list, x = [], 1
%o A249780 for n in range(2,10):
%o A249780     x = 2*x + 1
%o A249780     p = primefactors(x)
%o A249780     A249780_list.append(max(p)*min(p)) # _Chai Wah Wu_, Nov 05 2014
%K A249780 nonn
%O A249780 2,1
%A A249780 _Jacob Vecht_, Nov 05 2014
%E A249780 More terms from _Derek Orr_, Nov 05 2014