A390430 - OEIS

%I #11 Dec 07 2025 10:40:00

%S 0,2,4,2,6,3,8,2,4,4,10,3,12,5,5,2,14,3,16,4,6,6,18,3,6,7,4,5,20,4,22,

%T 2,7,8,7,3,24,9,8,4,26,5,28,6,5,10,30,3,8,4,9,7,32,3,8,5,10,11,34,4,

%U 36,12,6,2,9,6,38,8,11,5,40,3,42,13,5,9,9,7,44

%N Least prime index of n plus greatest prime index of n; a(1) = 0.

%C First differs from A360457 at a(42) = 5, A360457(42) = 4. The prime indices of 42 are {1,2,4}, with median 2.

%C A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798.

%e The prime indices of 42 are {1,2,4} so a(42) = 1 + 4 = 5.

%t prix[n_]:=If[n==1,{},Flatten[Cases[FactorInteger[n],{p_,k_}:>Table[PrimePi[p],{k}]]]];

%t Table[If[n==1,0,First[prix[n]]+Last[prix[n]]],{n,1,100}]

%Y For prime factors instead of indices we have A074320.

%Y For difference instead of sum we have A243055, row sums of A355536.

%Y For product instead of sum we have A333352.

%Y Positions of odd terms are A390988, complement A391226.

%Y Positions of first appearances appear to be A360006.

%Y A055396 gives least prime index.

%Y A061395 gives greatest prime index.

%Y A112798 lists prime indices, sum A056239.

%Y A390307 lists first sums of prime indices, see A390362, A390448, A390449, A390990.

%Y A390363 adds up greatest two prime indices.

%Y Cf. A124010, A243056, A286470, A355534, A390309.

%K nonn

%O 1,2

%A _Gus Wiseman_, Nov 29 2025