cp's OEIS Frontend

A061022 Primes of the form abbbbb... where a and b are digits.

%I A061022 #25 May 20 2025 21:33:34
%S A061022 2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,
%T A061022 97,199,211,233,277,311,433,499,577,599,677,733,811,877,911,977,1777,
%U A061022 1999,2111,2333,2777,2999,4111,4999,5333,7333,8111,8999,23333,47777
%N A061022 Primes of the form abbbbb... where a and b are digits.
%C A061022 Number of terms of n digits: 4, 21, 15, 12, 7, 8, 2, 7, 2, 3, 5, 2, 2, 7, 2, 4, 2, 2, 4, 3, 1, 0, 3, 3, 0, ..., . - _Robert G. Wilson v_, May 29 2011
%H A061022 Robert G. Wilson v, <a href="/A061022/b061022.txt">Table of n, a(n) for n = 1..262</a> (first 79 terms from Harry J. Smith) (shortened by _N. J. A. Sloane_, Jan 13 2019)
%e A061022 4111 is a member where a=4 and b = 1.
%p A061022 N:= 20:
%p A061022 A:= 2,3,5,7:
%p A061022 for n from 2 to N do
%p A061022   for a from 1 to 9 do
%p A061022     for b in [1,3,7,9] do
%p A061022        p:= a*10^(n-1) + b*(10^(n-1)-1)/9;
%p A061022        if isprime(p) then A:= A,p fi
%p A061022     od
%p A061022   od
%p A061022 od:
%p A061022 A;  # _Robert Israel_, Oct 13 2014
%t A061022 f[n_] := Select[ Union@ Flatten@ Table[ FromDigits@ Join[{a}, Table[b, {n - 1}]], {a, 9}, {b, {1, 3, 7, 9}}], PrimeQ]; Array[f, 5] // Flatten (* _Robert G. Wilson v_, May 29 2011 *)
%o A061022 (PARI) { n=r=0; default(primelimit, 1777777777); forprime (p=2, 1777777777, if (p>100, r=p\10; d=p-r*10; while (r>9 && r-r\10*10 == d, r\=10)); if (r<=9, write("b061022.txt", n++, " ", p)) ) } \\ _Harry J. Smith_, Jul 16 2009
%Y A061022 Cf. A034845, A062353.
%K A061022 nonn,base,easy
%O A061022 1,1
%A A061022 _Amarnath Murthy_, Jun 23 2001
%E A061022 Corrected and extended by _Dean Hickerson_, Jul 10 2001