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Denominator of Sum_{k=1..n} 1/lambda(k), where lambda(k) is the Carmichael's lambda function.
2

%I #13 Sep 19 2019 19:48:07

%S 1,1,2,1,4,4,12,12,12,6,15,30,60,60,15,60,240,80,720,720,720,720,7920,

%T 7920,1584,1584,1584,1584,11088,11088,55440,55440,55440,13860,6930,

%U 3465,13860,13860,6930,13860,27720,27720,27720,27720,27720,27720,637560,637560

%N Denominator of Sum_{k=1..n} 1/lambda(k), where lambda(k) is the Carmichael's lambda function.

%H Amiram Eldar, <a href="/A212716/b212716.txt">Table of n, a(n) for n = 1..10000</a>

%e 1, 2, 5/2, 3, 13/4, 15/4, 47/12, 53/12, 55/12, 29/6, 74/15, 163/30, ...

%p with(numtheory): a:=n->denom(sum(1/lambda(k), k=1..n)): seq(a(n), n=1..50);

%t Denominator[Table[Sum[1/CarmichaelLambda[k],{k,1,n}],{n,1,50}]]

%t Denominator @ Accumulate @ Table[1/CarmichaelLambda[n], {n, 1, 50}] (* _Amiram Eldar_, Sep 19 2019 *)

%Y Cf. A002322, A048049, A211306 (numerator).

%K nonn,frac

%O 1,3

%A _Michel Lagneau_, May 25 2012