A087274 - OEIS

%I #17 Jul 12 2024 10:16:42

%S 4,3,1,5,7,3,6,10,4,5,15,4,11,6,20,3,24,9,26,28,5,7,3,19,21,8,11,9,13,

%T 7,43,45,27,8,4,49,17,4,54,6,57,7,13,10,12,9,66,19,11,14,4,72,42,10,

%U 44,22,26,12,6,47,7,5,89,91,15,7,20,9,98,32,16,5,10,4,104,9,21,35,14,63,12,22

%N Prime index of the largest prime factor of 3*prime(n)+1.

%H Amiram Eldar, <a href="/A087274/b087274.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A000720(A006530(1+3*A000040(n))).

%F a(n) = A000720(A087273(n)). - _Amiram Eldar_, Jul 12 2024

%e n=10: prime(10)=29, max-p-factor(88)=11, pi(11)=5=a(10)<n;

%e n=11: prime(11)=31, max-p-factor(94)=47, pi(47)=15=a(11)>n;

%t ffi[x_] := Flatten[FactorInteger[x]]; ma[x_] := Part[Reverse[ffi[x]], 2]; Table[PrimePi[ma[3*Prime[w]+1]], {w, 1, 100}]

%o (PARI) a(n) = primepi(vecmax(factor(3*prime(n)+1)[, 1])); \\ _Michel Marcus_, Mar 27 2020

%Y Cf. A000720, A006530, A087272, A087273.

%K nonn

%O 1,1

%A _Labos Elemer_, Sep 18 2003