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%I #21 Sep 19 2023 12:39:32
%S 7,11,17,41,47,71,1117,1171,1447,1471,1741,1747,1777,4111,4177,4441,
%T 4447,7177,7411,7417,7477,7717,7741,11117,11171,11177,11411,11447,
%U 11471,11717,11777,14177,14411,14447,14717,14741,14747,14771,17117,17417
%N Prime numbers using only the straight digits 1, 4 and 7.
%C The number of decimal digits of a(n) is never divisible by 3. - _Robert Israel_, May 22 2014
%C The smallest prime using only all three straight digits is a(9) = 1447 (see Prime Curios! link). - _Bernard Schott_, Sep 08 2023
%H Robert Israel, <a href="/A079651/b079651.txt">Table of n, a(n) for n = 1..10000</a>
%H Chris K. Caldwell and G. L. Honaker, Jr., <a href="https://t5k.org/curios/page.php?short=1447">1447</a>, Prime Curios! [Gupta]
%e 17 is a term because it is a prime and consists of straight digits 1 and 7 only.
%p f:= proc(x) local n,d,t,i,a;
%p n:= floor(log[3]((2*x+3)));
%p if n mod 3 = 0 then return 0 fi;
%p d:=x - (3^n - 3)/2;
%p t:= 0;
%p for i from 0 to n-1 do
%p a:= d mod 3;
%p t:= t + (3*a+1)*10^i;
%p d:= (d-a)/3;
%p od:
%p t
%p end proc:
%p select(isprime, map(f, [$1..1000])); # _Robert Israel_, May 22 2014
%t Select[Prime[Range[2000]], Union[ Join[ IntegerDigits[ # ], {1, 4, 7}]] == {1, 4, 7} &]
%o (PARI) straight(n)=my(t);while(n,t=n%10;if(t!=1&&t!=4&&t!=7,return(0));n\=10);!!t
%o select(straight, primes(1000)) \\ _Charles R Greathouse IV_, Sep 25 2012
%Y Cf. A028373.
%K base,nonn
%O 1,1
%A _Shyam Sunder Gupta_, Jan 23 2003
%E Corrected and extended by _Robert G. Wilson v_, Jan 24 2003
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