A352260 Integers that need 2 iterations of the map x->A352172(x) to reach 1.

25, 52, 125, 152, 205, 215, 250, 251, 455, 502, 512, 520, 521, 545, 554, 1025, 1052, 1125, 1152, 1205, 1215, 1250, 1251, 1455, 1502, 1512, 1520, 1521, 1545, 1554, 2005, 2015, 2050, 2051, 2105, 2115, 2150, 2151, 2255, 2500, 2501, 2510, 2511, 2525, 2552, 4055, 4155, 4505

Offset

1,1

Links

Table of n, a(n) for n=1..48.

Example

25 -> 1000 -> 1.

Mathematica

f[n_] := (Times @@ Select[IntegerDigits[n], # > 1 &])^3; q[n_, len_] := (v = Nest[f, n, len - 1]) != 1 && f[v] == 1; Select[Range[4505], q[#, 2] &] (* Amiram Eldar, Mar 10 2022 *)

Prog

(PARI) f(n) = vecprod(apply(x->x^3, select(x->(x>1), digits(n)))); \\ A352172
isok2(n) = {for (k=1, 2, n = f(n); if ((n==1), return(k==2)); ); }
(Python)
from math import prod
def A352172(n): return prod(int(d)**3 for d in str(n) if d != '0')
def ok(x):
    x = A352172(x)
    return x != 1 and A352172(x) == 1
print([k for k in range(4506) if ok(k)]) # Michael S. Branicky, Mar 10 2022

Crossrefs

Cf. A352172. Subsequence of A351876.

Cf. A352261, A352262, A352263, A352264, A352265, A352266, A352267, A352268.

Sequence in context: A042248 A132766 A039412 * A199990 A201057 A043235

Adjacent sequences: A352257 A352258 A352259 * A352261 A352262 A352263

Keyword

nonn,base

Author

Michel Marcus, Mar 10 2022

Status

approved