A221220 Numbers with more than one prime factor such that concatenation of its prime factors (without multiplicity) is a prime.

6, 12, 18, 21, 22, 24, 33, 36, 39, 44, 46, 48, 51, 54, 58, 63, 66, 70, 72, 82, 88, 92, 93, 96, 99, 108, 111, 115, 116, 117, 132, 133, 140, 141, 142, 144, 147, 153, 154, 159, 162, 164, 165, 166, 176, 177, 182, 184, 187, 189, 192, 198, 201, 205, 210, 216, 219

Offset

1,1

Links

Table of n, a(n) for n=1..57.

Example

Prime factors of 140 are 2, 5, and 7 and 257 is prime, so 140 is a term.

Mathematica

Select[Range[220], Length[x=First/@FactorInteger[#]]>1&&PrimeQ[FromDigits[Flatten[IntegerDigits[x]]]]&]

Prog

(Python)
from sympy import isprime, primefactors
def ok(n):
    pf = primefactors(n)
    if len(pf) < 2: return False
    return isprime(int("".join(str(p) for p in pf)))
print(list(filter(ok, range(2, 220)))) # Michael S. Branicky, Jun 12 2021

Crossrefs

Cf. A024619, A046411.

Sequence in context: A348719 A316221 A138939 * A046411 A364348 A364537

Adjacent sequences: A221217 A221218 A221219 * A221221 A221222 A221223

Keyword

nonn,base,less

Author

Jayanta Basu, Jun 04 2013

Status

approved