A034821 - OEIS

A034821

Concatenations C1 and C2 are both prime (see the comment lines).

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

0,1

COMMENTS

C1 = 'prevprime(n) followed by nextprime(n)'

C2 = 'nextprime(n) followed by prevprime(n)'

LINKS

Table of n, a(n) for n=0..51.

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

Cf. A034808-A034820.

Sequence in context: A145986 A375424 A200992 * A158007 A349011 A143562

Adjacent sequences: A034818 A034819 A034820 * A034822 A034823 A034824

KEYWORD

nonn,base

AUTHOR

Patrick De Geest, Oct 15 1998

STATUS

approved