cp's OEIS Frontend

A210720 Primes formed by concatenating n, n, n, n, and 1 for n = 1, 2, 3,....

%I A210720 #31 Oct 26 2024 04:57:42
%S A210720 33331,99991,242424241,404040401,454545451,464646461,494949491,
%T A210720 525252521,575757571,737373731,787878781,949494941,1021021021021,
%U A210720 1081081081081,1091091091091,1211211211211,1291291291291,1481481481481,1511511511511
%N A210720 Primes formed by concatenating n, n, n, n, and 1 for n = 1, 2, 3,....
%C A210720 This is to four (duplicated strings concatenated) as A210712 is to three (duplicated strings concatenated), and as A210511 is to two (duplicated strings concatenated).
%H A210720 Vincenzo Librandi, <a href="/A210720/b210720.txt">Table of n, a(n) for n = 1..1000</a>
%e A210720 a(1) = 33331 because Concat(3,3,3,1) = 3331 which is in A000040.
%p A210720 A210720 := proc(n)
%p A210720         local p;
%p A210720         [n,n,n,n,1] ;
%p A210720         p := digcatL(%) ;
%p A210720         if isprime(p) then
%p A210720                 printf("%d,",p) ;
%p A210720         end if;
%p A210720 end proc:
%p A210720 for n from 1 to 400 do
%p A210720         A210720(n) ;
%p A210720 end do: # _R. J. Mathar_, Feb 10 2013
%t A210720 Select[Table[FromDigits[Flatten[{IntegerDigits[n], IntegerDigits[n], IntegerDigits[n], IntegerDigits[n], IntegerDigits[1], {}}]], {n, 200}], PrimeQ] (* _Vincenzo Librandi_, Mar 15 2013 *)
%t A210720 Select[Table[FromDigits[Join[Flatten[IntegerDigits/@PadRight[{},4,n]],{1}]],{n,200}],PrimeQ] (* _Harvey P. Dale_, Oct 16 2017 *)
%o A210720 (Magma) [nnnn1: n in [1..200] | IsPrime(nnnn1) where nnnn1 is Seqint([1] cat Intseq(n) cat Intseq(n) cat Intseq(n) cat Intseq(n))]; // _Vincenzo Librandi_, Mar 15 2013
%Y A210720 Cf. A210511, A210712.
%K A210720 nonn,base,easy
%O A210720 1,1
%A A210720 _Jonathan Vos Post_, Jan 29 2013