A355550 - OEIS

%I #9 Jul 07 2022 05:03:22

%S 0,4,16,1300,976,40300,662704,12251200

%N a(n) is the smallest number k such that k occurs in the Reverse-and-Multiply trajectories of exactly n positive integers less than k.

%C Smallest k such that A355549(k) = n.

%o (PARI) revmult(n) = n*eval(concat(Vecrev(Str(n))))

%o a355549(n) = my(i=0); for(k=2, n-1, my(x=k); while(x < n, if(x==revmult(x), break, x=revmult(x))); if(x==n, i++)); i

%o a(n) = for(k=0, oo, if(a355549(k)==n, return(k)))

%Y Cf. A355548, A355549.

%K nonn,base,hard,more

%O 0,2

%A _Felix Fröhlich_, Jul 06 2022

%E a(5)-a(7) from _Jinyuan Wang_, Jul 07 2022