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 A084670 Numbers k such that concatenation of prime(k) and k is prime. 2
 3, 9, 19, 21, 37, 63, 77, 81, 87, 107, 121, 133, 177, 201, 211, 213, 217, 281, 293, 303, 321, 327, 329, 333, 351, 391, 393, 439, 481, 503, 507, 519, 543, 547, 551, 561, 579, 581, 599, 621, 639, 657, 663, 667, 711, 721, 727, 743, 793, 813, 819, 827, 829, 831, 837 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 EXAMPLE 9 is a term because prime(9) = 23 and concatenation of 23 and 9 is prime MATHEMATICA Select[Range[1000], PrimeQ[Prime[#]10^IntegerLength[#]+#]&] (* Harvey P. Dale, Apr 09 2022 *) PROG (PARI) is(k) = ispseudoprime(eval(Str(prime(k), k))); \\ Jinyuan Wang, Apr 10 2020 (Python) from sympy import isprime, prime def aupto(lim):   return [k for k in range(1, lim+1) if isprime(int(str(prime(k))+str(k)))] print(aupto(837)) # Michael S. Branicky, Mar 09 2021 CROSSREFS Cf. A075110, A084669. Sequence in context: A253219 A097267 A043097 * A002091 A056259 A056682 Adjacent sequences:  A084667 A084668 A084669 * A084671 A084672 A084673 KEYWORD nonn,base AUTHOR Zak Seidov, Jun 29 2003 EXTENSIONS More terms from Jinyuan Wang, Apr 10 2020 STATUS approved

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Last modified May 23 06:27 EDT 2022. Contains 353961 sequences. (Running on oeis4.)