OFFSET
1,1
COMMENTS
In general rebase notation (Marc LeBrun): p7 = (7) [p] (10).
LINKS
Harry J. Smith, Table of n, a(n) for n = 1..1000
EXAMPLE
E.g., 787_10 = 2203_7 is prime, and so is 2203_10.
MATHEMATICA
Select[ Range[2500], PrimeQ[ # ] && PrimeQ[ FromDigits[ IntegerDigits[ #, 7]]] & ]
Select[Prime[Range[300]], PrimeQ[FromDigits[IntegerDigits[#, 7]]]&] (* Harvey P. Dale, Nov 10 2022 *)
PROG
(PARI) baseE(x, b)= { local(d, e=0, f=1); while (x>0, d=x-b*(x\b); x\=b; e+=d*f; f*=10); return(e) } { n=0; for (m=1, 10^9, p=prime(m); b=baseE(p, 7); if (isprime(b), write("b065725.txt", n++, " ", p); if (n==1000, return)) ) } \\ Harry J. Smith, Oct 27 2009
(PARI) isok(p) = isprime(p) && isprime(fromdigits(digits(p, 7), 10)); \\ Michel Marcus, Mar 05 2022
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, Nov 15 2001
STATUS
approved