A249780 - OEIS

%I #16 May 23 2020 13:38:44

%S 9,49,15,961,21,16129,51,511,93,2047,39,67092481,381,1057,771,

%T 17179607041,219,274876858369,123,2359,2049,8388607,723,55831,24573,

%U 1838599,381,486737,993,4611686014132420609,196611,4196353,393213,3810551,327,137438953471,1572861,849583,185043

%N Product of lowest and highest prime factors of 2^n-1

%H Chai Wah Wu, <a href="/A249780/b249780.txt">Table of n, a(n) for n = 2..200</a>

%F a(n) = A005420(n) * A049479(n)

%e The lowest and higest prime factors of 2^6-1 are 3 and 7, so A(6) = 21

%p a:= proc(n) local F; F:= numtheory:-factorset(2^n-1); min(F)*max(F) end proc:

%p seq(a(n),n=2..50); # _Robert Israel_, Nov 05 2014

%t 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 (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 (Python)

%o from sympy import primefactors

%o A249780_list, x = [], 1

%o for n in range(2,10):

%o ....x = 2*x + 1

%o ....p = primefactors(x)

%o ....A249780_list.append(max(p)*min(p)) # _Chai Wah Wu_, Nov 05 2014

%K nonn

%O 2,1

%A _Jacob Vecht_, Nov 05 2014

%E More terms from _Derek Orr_, Nov 05 2014