A091439 - OEIS

%I #12 Jan 13 2020 11:29:15

%S 1,2,6,210,30030,223092870,13082761331670030,

%T 3217644767340672907899084554130,

%U 1492182350939279320058875736615841068547583863326864530410

%N Least k such that k/phi(k) >= n, where phi is Euler's totient function.

%C These are all primorial numbers (A002110).

%C It appears that the primorial index of a(n) is A256968(n-1) for n>=1. - _Michel Marcus_, Oct 07 2019

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/TotientFunction.html">Totient Function</a>

%t Table[prod=1; i=0; While[prod<n, i++; prod=prod/(1-1/Prime[i])]; Times@@Prime[Range[i]], {n, 1, 9}]

%o (PARI) a(n) = my(p=1, i=0); while(p<n, i++; p = p/(1-1/prime(i))); prod(k=1, i, prime(k)); \\ _Michel Marcus_, Oct 07 2019

%Y Cf. A002110, A091456 (n * phi(k) < k).

%K easy,nonn

%O 1,2

%A _T. D. Noe_, Jan 09 2004