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A066737
Composite numbers that are concatenations of primes.
5
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
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
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