A391844 - OEIS

%I #7 Jan 16 2026 13:17:12

%S 1,1,0,0,0,1,0,0,0,1,0,0,0,1,1,0,0,1,0,0,1,1,0,0,0,1,0,0,0,1,0,0,1,1,

%T 1,0,0,1,1,0,0,1,0,0,0,1,0,0,0,1,1,0,0,1,0,0,1,1,0,0,0,1,0,0,1,1,0,0,

%U 1,1,0,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,1,0,0,1,0,0,1,1,1,0,0,1,0,0,0,1,0,0,0,1,0,0,0,1,1

%N a(n) = 1 if the least prime not dividing the arithmetic derivative of n is equal to the least prime dividing n, otherwise 0. a(1) = 1 by convention.

%H Antti Karttunen, <a href="/A391844/b391844.txt">Table of n, a(n) for n = 1..65537</a>

%H <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>.

%o (PARI)

%o A003415(n) = if(n<=1, 0, my(f=factor(n)); n*sum(i=1, #f~, f[i, 2]/f[i, 1]));

%o A020639(n) = if(1==n,n,vecmin(factor(n)[, 1]));

%o A053669(n) = forprime(p=2, , if(n%p, return(p))); \\ From A053669

%o A391844(n) = if(n<2,n,(A053669(A003415(n))==A020639(n)));

%Y Characteristic function of A391845.

%Y Cf. A003415, A020639, A053669.

%Y Cf. also A370124.

%K nonn

%O 1

%A _Antti Karttunen_, Jan 16 2026