A121608 Primes that can be written as concatenation of two composite numbers in decimal representation.

89, 109, 149, 229, 269, 349, 359, 389, 409, 421, 433, 439, 449, 457, 463, 487, 491, 499, 509, 569, 659, 677, 691, 709, 769, 809, 821, 827, 829, 839, 857, 859, 863, 877, 881, 887, 919, 929, 977, 991, 1009, 1021, 1033, 1039, 1049, 1051, 1063, 1069, 1087, 1091

Offset

1,1

Links

Michael S. Branicky, Table of n, a(n) for n = 1..10000

Example

A000040(169) = 1009 = 100*10+9 = A002808(74)*10+A002808(4), therefore 1009 is a term: a(41) = 1009;
A000040(172) = 1021 = 10*100+21 = A002808(5)*100+A002808(12), therefore 1021 is a term: a(42) = 1021.

Prog

(Python)
from sympy import isprime
def comp(s): i=int(s); return s[0]!='0' and i > 1 and not isprime(i)
def ok(n):
  s = str(n)
  for i in range(1, len(s)):
    if comp(s[:i]) and comp(s[i:]) and isprime(int(s)): return True
print([m for m in range(1092) if ok(m)]) # Michael S. Branicky, Feb 27 2021

Crossrefs

Cf. A105184, A121609, A121610.

Sequence in context: A050956 A352540 A350724 * A135144 A083371 A124583

Adjacent sequences: A121605 A121606 A121607 * A121609 A121610 A121611

Keyword

nonn,base

Author

Reinhard Zumkeller, Aug 10 2006

Status

approved