The On-Line Encyclopedia of Integer Sequences (OEIS)

Revision History for A020455 (Bold, blue-underlined text is an addition; faded, red-underlined text is a ~~deletion~~.)
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A020455

Primes that contain digits 1 and 7 only.
(history; published version)

#39 by Charles R Greathouse IV at Thu Sep 08 08:44:45 EDT 2022

PROG

(MAGMAMagma) [p: p in PrimesUpTo(1771177) | Set(Intseq(p)) subset [1, 7]]; // Vincenzo Librandi, Jul 27 2012

Discussion

Thu Sep 08

08:44

OEIS Server: https://oeis.org/edit/global/2944

#38 by Joerg Arndt at Sun Jul 11 07:21:40 EDT 2021

STATUS

proposed

approved

#37 by Michael S. Branicky at Sun Jul 11 07:15:48 EDT 2021

STATUS

editing

proposed

#36 by Michael S. Branicky at Sun Jul 11 07:11:54 EDT 2021

DATA

7, 11, 17, 71, 1117, 1171, 1777, 7177, 7717, 11117, 11171, 11177, 11717, 11777, 17117, 71171, 71711, 71777, 77171, 77711, 1111711, 1111771, 1117111, 1117117, 1117177, 1171111, 1171117, 1171771, 1177171, 1177711, 1177717, 1711117, 1717117, 1771177, 1771717

#35 by Michael S. Branicky at Sun Jul 11 07:10:02 EDT 2021

PROG

(Python)

from sympy import isprime

def only17(n): return int(bin(n+1)[3:].replace('1', '7').replace('0', '1'))

def auptod(digs):

return list(filter(isprime, (only17(i) for i in range(1, 2**(digs+1)-1))))

print(auptod(8)) # Michael S. Branicky, Jul 11 2021

STATUS

approved

editing

#34 by Bruno Berselli at Mon Mar 29 10:39:10 EDT 2021

STATUS

reviewed

approved

#33 by Joerg Arndt at Sun Mar 28 00:44:11 EDT 2021

STATUS

proposed

reviewed

#32 by Mikk Heidemaa at Sat Mar 27 15:29:03 EDT 2021

STATUS

editing

proposed

#31 by Mikk Heidemaa at Sat Mar 27 15:22:50 EDT 2021

COMMENTS

Every There are no terms whose number of digits is divisible by 3: for every d that is a multiple of 3, every d-digit number j consisting of no digits other than 1's and 7's is divisible by 3 if its number of digits is will have a digit sum divisible by 3, so there are no terms whose number of digits is j will also be divisible by 3. - Mikk Heidemaa, Mar 26 2021

#30 by Mikk Heidemaa at Sat Mar 27 14:40:04 EDT 2021

COMMENTS

Every number consisting of no digits other than 1's and 7's is divisible by 3 if its number of digits is divisible by 3, so this sequence contains there are no terms whose number of digits is divisible by 3. - Mikk Heidemaa, Mar 26 2021

STATUS

proposed

editing

Discussion

Sat Mar 27

14:48

Mikk Heidemaa: That (at 14:13) looks good.