cp's OEIS Frontend

A247025 Lengths of prefixes of the infinite string of digits repeat(1379) that are prime.

%I A247025 #41 Jul 10 2025 19:39:47
%S A247025 2,3,7,81,223,250,255,537,543,1042,2103,4285,25015,35361,43525
%N A247025 Lengths of prefixes of the infinite string of digits repeat(1379) that are prime.
%C A247025 Every prime > 5 in base 10 ends in 1, 3, 7, or 9.  If those digits are repeated, in order, some prefixes of that string are prime.
%C A247025 n such that floor(1379/9999 * 10^n) is prime. - _Robert Israel_, Sep 09 2014
%C A247025 a(13) > 15500. - _Daniel Starodubtsev_, Mar 16 2021
%e A247025 1 and 3 are the first two digits of the string, and 13 is prime. 13 has length 2, so 2 is a term.
%e A247025 137 is prime and three digits long, so 3 is a term.
%e A247025 1379137 is prime and seven digits long, so 7 is a term.
%t A247025 Select[Range[4300],PrimeQ[FromDigits[PadRight[{},#,{1,3,7,9}]]]&] (* The program generates the first 12 terms of the sequence. *) (* _Harvey P. Dale_, Jun 11 2024 *)
%o A247025 (Python)
%o A247025 from sympy import isprime
%o A247025 from itertools import cycle
%o A247025 it=cycle([1,3,7,9])
%o A247025 c=0
%o A247025 a=0
%o A247025 for i in it:
%o A247025     c+=1
%o A247025     a*=10
%o A247025     a+=i
%o A247025     if isprime(a):
%o A247025         print(c)
%o A247025 (PARI) lista(nn) = {s = 0; digs = [1,3,7,9]; id = 1; for (n=1, nn, s = 10*s + digs[id]; if (isprime(s), print1(n, ", ")); id++; if (id==5, id = 1););} \\ _Michel Marcus_, Oct 11 2014
%o A247025 (Magma) [n: n in [0..300] | IsPrime(Floor(1379/9999 * 10^n))]; // _Vincenzo Librandi_, Oct 17 2014
%Y A247025 Cf. A000040, A007652.
%K A247025 nonn,base,more,less
%O A247025 1,1
%A A247025 _Mark E. Shoulson_, Sep 09 2014
%E A247025 Edited. Name specified. Example reformulated. a(12) added (using R. Israel's formula). Keyword less and Crossreferences added. - _Wolfdieter Lang_, Nov 03 2014
%E A247025 a(13)-a(14) from _Michael S. Branicky_, May 29 2023
%E A247025 a(15) from _Michael S. Branicky_, Jun 13 2024