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A034821
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Concatenations C1 and C2 are both prime (see the comment lines).
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13
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5, 17, 103, 127, 157, 191, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 227, 234, 235, 236, 237, 238, 258, 259, 260, 261, 262, 307, 347, 354, 355, 356, 357, 358, 431, 524, 525, 526, 527, 528, 529, 530, 531, 532, 533, 534, 535, 536, 537, 538, 539
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OFFSET
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0,1
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COMMENTS
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C1 = 'prevprime(n) followed by nextprime(n)'
C2 = 'nextprime(n) followed by prevprime(n)'
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LINKS
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EXAMPLE
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n=17 -> next prime is 19, previous prime is 13, thus '1319' and '1913' are both primes.
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MATHEMATICA
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c1c2pQ[n_]:=Module[{c1=NextPrime[n, -1], c2=NextPrime[n]}, AllTrue[ {c1* 10^IntegerLength[ c2]+c2, c2*10^IntegerLength[c1]+c1}, PrimeQ]]; Select[ Range[600], c1c2pQ] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 23 2019 *)
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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