A087274 Prime index of the largest prime factor of 3*prime(n)+1.

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, 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, 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

Offset: 1

Author

Labos Elemer, Sep 18 2003

Keywords

• nonn

Examples

			n=10: prime(10)=29, max-p-factor(88)=11, pi(11)=5=a(10)<n;
n=11: prime(11)=31, max-p-factor(94)=47, pi(47)=15=a(11)>n;
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000720, A006530, A087272, A087273.

Programs

Mathematica

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

PARI

a(n) = primepi(vecmax(factor(3*prime(n)+1)[, 1])); \\ Michel Marcus, Mar 27 2020

Formula

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

a(n) = A000720(A087273(n)). - Amiram Eldar, Jul 12 2024