A073741 - OEIS

%I #12 Dec 15 2017 17:35:59

%S 1,1,2,2,1,1,1,1,2,2,3,3,1,1,4,4,1,1,1,1,2,2,2,2,1,1,2,2,1,1,3,3,2,2,

%T 1,1,5,5,4,4,3,3,1,1,8,8,1,1,1,1,6,6,1,1,1,1,2,2,2,2,7,7,2,2,5,5,1,1,

%U 2,2,3,3,1,1,4,4,1,1,3,3,10,10,2,2,1,1,2,2,1,1,7,7,582,582,1,1,1,1,2,2,2,2

%N Number of steps needed to reach a prime when the following map is repeatedly applied to n: if n is even then 2n+1, otherwise 2n-1; or -1 if no prime is ever reached.

%e For n=4, a(4)=2 because 4 ->9 ->17

%t pr[n_]:=Module[{c=If[EvenQ[n],2n+1,2n-1]},Length[NestWhileList[ If[EvenQ[#], 2#+1, 2#-1]&, c,!PrimeQ[#]&]]]; Array[pr,102,2] (* _Harvey P. Dale_, Oct 05 2012 *)

%o (UBASIC) 10 cls 30 for I=2 to 100 32 H=I 40 if odd(H)=1 then goto 90 else goto 50 50 A=2*H+1:K=K+1 60 if prmdiv(A)=A then print I,K:goto 120 65 if K>10000 then print I,"no solution":goto 120 70 H=A:goto 40 90 A=2*H-1:K=K+1 100 if prmdiv(A)=A then print I,K:goto 120 105 if K>10000 then print I,"no solution":goto 120 110 H=A:goto 40 120 K=0 130 next

%K easy,nonn

%O 2,3

%A _Felice Russo_, Sep 02 2002

%E More terms from _Jason Earls_, Sep 04 2002