|
|
A020455
|
|
Primes that contain digits 1 and 7 only.
|
|
11
|
|
|
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
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,1
|
|
COMMENTS
|
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 will have a digit sum divisible by 3, so j will also be divisible by 3. - Mikk Heidemaa, Mar 26 2021
|
|
LINKS
|
|
|
FORMULA
|
|
|
MATHEMATICA
|
Flatten[Table[Select[FromDigits/@Tuples[{1, 7}, n], PrimeQ], {n, 7}]] (* Vincenzo Librandi, Jul 27 2012 *)
|
|
PROG
|
(Magma) [p: p in PrimesUpTo(1771177) | Set(Intseq(p)) subset [1, 7]]; // Vincenzo Librandi, Jul 27 2012
(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))))
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn,base
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|