A066737 Composite numbers that are concatenations of primes.

22, 25, 27, 32, 33, 35, 52, 55, 57, 72, 75, 77, 112, 115, 117, 132, 133, 135, 172, 175, 177, 192, 195, 213, 217, 219, 222, 225, 231, 232, 235, 237, 243, 247, 252, 253, 255, 259, 261, 267, 272, 273, 275, 279, 289, 292, 295, 297, 312, 315, 319, 322, 323, 325

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Formula

A066737 = A152242 \ A000040 = A152242 intersect A002808. - M. F. Hasler, Oct 16 2009

Example

72 is the concatenation of primes 7 and 2. 132 is the concatenation of primes 13 and 2. 225 is the concatenation of 2, 2 and 5.

Maple

ccat:= proc(m, n) 10^(1+ilog10(n))*m+n end proc:
C[1]:= {2, 3, 5, 7}: P[1]:=C[1]:
for n from 2 to 3 do
  P[n]:= select(isprime, {seq(i, i=10^(n-1)+1..10^n-1, 2)});
  C[n]:= P[n];
  for m from 1 to n-1 do
    C[n]:= C[n] union {seq(seq(ccat(p, q), p =P[m]), q=C[n-m])};
  od
od:
seq(op(sort(convert(remove(isprime, C[n]), list))), n=1..3); # Robert Israel, Jan 22 2020

Prog

(PARI) for(n=1, 999, !isprime(n) && is_A152242(n) && print1(n", ")) \\ M. F. Hasler, Oct 16 2009

Crossrefs

Cf. A121609.

Sequence in context: A186780 A034304 A167459 * A121609 A092631 A061371

Adjacent sequences: A066734 A066735 A066736 * A066738 A066739 A066740

Keyword

base,easy,nonn

Author

Joseph L. Pe, Jan 15 2002

Extensions

More terms from Larry Reeves (larryr(AT)acm.org), Apr 03 2002

Missing terms added by M. F. Hasler, Oct 16 2009

Status

approved