# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a210720 Showing 1-1 of 1 %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, Table of n, a(n) for n = 1..1000 %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 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE