A101700 - OEIS

%I #6 Mar 23 2022 15:19:18

%S 21,291,2991,299999999999999991

%N Numbers of the form 3*(10^n-3), where 10^n-3 is prime.

%C a(5) = 3*(10^140-3) is 141 digits long and is too large to include.

%C If m is in this sequence then phi(m)=r(m), so this sequence is a subsequence of A069215. a(n)=3*(10^A089675(n)-3), so a(4)=3*(10^17-3), a(5)=3*(10^140-3), a(6)=3*(10^990-3), a(7)=3*(10^1887-3), a(8)=3*(10^3530-3), a(9)=3*(10^5996-3), a(10)=3*(10^13820-3), a(11)=3*(10^21873-3) & a(12)=3*(10 ^26045-3).

%F a(n) = 3*(10^A089675(n) - 3).

%e 299999999999999991 is in the sequence because (10^17-3) is prime and 3*(10^17-3)=299999999999999991.

%t Do[If[PrimeQ[10^n-3], Print[3*(10^n-3)]], {n, 150}]

%t 3#&/@Select[10^Range[20]-3,PrimeQ] (* _Harvey P. Dale_, Mar 23 2022 *)

%Y Cf. A086947, A089675, A069215.

%K nonn

%O 1,1

%A _Farideh Firoozbakht_, Dec 31 2004