cp's OEIS Frontend

A076425 Numbers n such that zero is never reached by iterating the mapping k -> abs(reverse(lpd(k))-reverse(gpf(k))). lpd(k) is the largest proper divisor and gpf(k) is the largest prime factor of k.

%I A076425 #8 May 30 2014 09:25:42
%S A076425 2074,2113,2179,2914,3111,4112,4371,4390,4456,4956,4978,5185,5450,
%T A076425 5750,6474,6585,6827,7248,7259,7285,7467,8175,8625,8647,9378,9711,
%U A076425 9739,10199,10975,11407,11752,12006,12232,12338,12445,12826,13224,13396
%N A076425 Numbers n such that zero is never reached by iterating the mapping k -> abs(reverse(lpd(k))-reverse(gpf(k))). lpd(k) is the largest proper divisor and gpf(k) is the largest prime factor of k.
%C A076425 n such that A076423(n) = -1.
%e A076425 For 4112 the mapping leads to a fixed point (cf. A076426): 4112 -> 5750 -> 5750 -> ...; for 2074 the mapping leads to a cycle: 2074 -> 7285 -> 7467 -> 9711 -> 7285 -> ...
%o A076425 (PARI) {stop=20; for(n=1,13600,c=1; b=1; k=n; while(b&&c<stop,v=divisors(k); a=matsize(v)[2]; z=if(a>1,v[a-1],1); p=0; while(z>0,d=divrem(z,10); z=d[1]; p=10*p+d[2]); z=if(k==1,1,vecmax(component(factor(k),1))); q=0; while(z>0,d=divrem(z,10); z=d[1]; q=10*q+d[2]); a=abs(p-q); if(a==0,b=0,k=a; c++)); if(a>0,print1(n,",")))}
%Y A076425 Cf. A076423, A076426.
%K A076425 base,nonn
%O A076425 1,1
%A A076425 _Klaus Brockhaus_, Oct 11 2002