This site is supported by donations to The OEIS Foundation.

 Annual Appeal: Please make a donation to keep the OEIS running. In 2018 we replaced the server with a faster one, added 20000 new sequences, and reached 7000 citations (often saying "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A295395 Numbers k such that (61*10^k - 1)/3 is prime. 0
 3, 17, 18, 30, 78, 98, 371, 947, 1085, 1560, 1607, 2145, 4310, 4637, 16674, 17148, 25209, 27929, 33491, 74189 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers such that the digits 20 followed by k occurrences of the digit 3 is prime (see Example section). a(21) > 10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 203w EXAMPLE 3 is in this sequence because (61*10^3 - 1)/3 = 20333 is prime. Initial terms and primes associated: a(1) = 3, 20333; a(2) = 17, 2033333333333333333; a(3) = 18, 20333333333333333333; a(4) = 30, 20333333333333333333333333333333;  etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(61*10^# - 1)/3] &] PROG (PARI) isok(k) = isprime((61*10^k - 1)/3); \\ Michel Marcus, Nov 22 2017 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A059189 A043066 A293764 * A246143 A101144 A038886 Adjacent sequences:  A295392 A295393 A295394 * A295396 A295397 A295398 KEYWORD nonn,more,hard AUTHOR Robert Price, Nov 21 2017 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 16 09:34 EST 2018. Contains 318160 sequences. (Running on oeis4.)