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A034821
Concatenations C1 and C2 are both prime (see the comment lines).
13
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
OFFSET
0,1
COMMENTS
C1 = 'prevprime(n) followed by nextprime(n)'
C2 = 'nextprime(n) followed by prevprime(n)'
EXAMPLE
n=17 -> next prime is 19, previous prime is 13, thus '1319' and '1913' are both primes.
MATHEMATICA
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 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Oct 15 1998
STATUS
approved