A042938 Reverse or triple: if reverse(a(n)) > a(n), a(n+1) = reverse(a(n)), else a(n+1) = 3*a(n).

1, 3, 9, 27, 72, 216, 612, 1836, 6381, 19143, 34191, 102573, 375201, 1125603, 3065211, 9195633, 27586899, 99868572, 299605716, 617506992, 1852520976, 6790252581, 20370757743, 34775707302, 104327121906, 609121723401, 1827365170203, 3020715637281, 9062146911843

Offset: 1

Author

Dann Toliver

Keywords

• base
• nonn

Examples

			Starting with 1, the reverse is 1 and not greater than 1 so the next term is 3. The reverse of 3 is also not greater than 3 and so the next term is 9. The reverse of 9 is not greater than 9 and so the next term is 27. However, the reverse of 27 is 72 which is greater and so the next term in the sequence is 72 and so on.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..1951

Programs

Maple

reverse:= proc(n)
local L,i,m;
L:= convert(n,base,10);
  m:= nops(L);
add(L[i]*10^(m-i),i=1..m);
end proc:
a[1]:= 1:
for n from 2 to 100 do
r:= reverse(a[n-1]);
if r > a[n-1] then a[n]:= r
else a[n]:= 3*a[n-1]
fi
od:
seq(a[i],i=1..100); # Robert Israel, Jun 24 2015

Mathematica

rd[n_]:=Module[{rev=FromDigits[Reverse[IntegerDigits[n]]]}, If[rev>n, rev, 3 n]]; NestList[rd, 1, 30] (* Vincenzo Librandi, Jun 24 2015 *)
NestList[If[IntegerReverse[#]>#,IntegerReverse[#],3#]&,1,30] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jul 09 2018 *)

PARI

lista(nn) = {print1(a=1, ", "); for (n=2, nn, r = eval(concat(Vecrev(Str(a)))); if (r > a, a = r, a *= 3); print1(a, ", "););} \\ Michel Marcus, Jan 31 2016

Extensions

Corrected by Robert Israel, Jun 24 2015