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A066737
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Composite numbers that are concatenations of primes.
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5
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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
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OFFSET
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1,1
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LINKS
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FORMULA
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EXAMPLE
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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.
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MAPLE
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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
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PROG
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(PARI) for(n=1, 999, !isprime(n) && is_A152242(n) && print1(n", ")) \\ M. F. Hasler, Oct 16 2009
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CROSSREFS
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KEYWORD
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base,easy,nonn
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AUTHOR
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EXTENSIONS
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More terms from Larry Reeves (larryr(AT)acm.org), Apr 03 2002
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STATUS
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approved
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