cp's OEIS Frontend

A351078 First noncomposite number reached when iterating the map x -> x', when starting from x = n, or 0 if no such number is ever reached. Here x' is the arithmetic derivative of x, A003415.

%I A351078 #18 Feb 12 2022 17:46:34
%S A351078 0,1,2,3,0,5,5,7,0,5,7,11,0,13,5,0,0,17,7,19,0,7,13,23,0,7,0,0,0,29,
%T A351078 31,31,0,5,19,0,0,37,7,0,0,41,41,43,0,0,7,47,0,5,0,0,0,53,0,0,0,13,31,
%U A351078 59,0,61,5,0,0,7,61,67,0,0,59,71,0,73,0,0,0,7,71,79,0,0,43,83,0,13,0,0,0,89,0,0,0,19,5
%N A351078 First noncomposite number reached when iterating the map x -> x', when starting from x = n, or 0 if no such number is ever reached. Here x' is the arithmetic derivative of x, A003415.
%C A351078 Primes of A189483 occur only once, on the corresponding indices, while A189441 may also occur in other positions.
%C A351078 There are interesting white "filament-like regions" in the scatter plot.
%H A351078 Antti Karttunen, <a href="/A351078/b351078.txt">Table of n, a(n) for n = 0..65537</a>
%F A351078 For all n, a(4*n) = a(27*n) = a((p^p)*n) = a(A099309(n)) = 0.
%F A351078 a(p) = p for all primes p.
%e A351078 For n = 15, if we iterate with A003415, we get a path 15 -> 8 -> 12 -> 16 -> 32 -> 80 -> 176 -> 368 -> ..., where the terms just keep on growing without ever reaching a prime or 1, therefore a(15) = 0.
%e A351078 For n = 18, its path down to zero, when iterating A003415 is: 18 -> 21 -> 10 -> 7 -> 1 -> 0, and the first noncomposite term on the path is prime 7, therefore a(18) = 7.
%o A351078 (PARI)
%o A351078 A003415checked(n) = if(n<=1, 0, my(f=factor(n), s=0); for(i=1, #f~, if(f[i,2]>=f[i,1],return(0), s += f[i, 2]/f[i, 1])); (n*s));
%o A351078 A351078(n) = { while(n>1&&!isprime(n), n = A003415checked(n)); (n); };
%Y A351078 Cf. A003415, A189441, A189483, A351079, A351259 [= a(A351255(n))].
%Y A351078 Cf. A099309 (positions of zeros after the initial one at a(0)=0), A328115 (positions of 5's), A328117 (positions of 7's).
%Y A351078 Cf. also A327968.
%K A351078 nonn,look
%O A351078 0,3
%A A351078 _Antti Karttunen_, Feb 11 2022