login
Primes whose base-9 representation also is the base-2 representation of a prime.
2

%I #31 Apr 14 2021 05:27:46

%S 739,811,6571,59779,532261,591301,4783699,4789621,4842109,4849399,

%T 5314411,5314501,5373469,5374279,43047541,43112341,43113061,47888821,

%U 47889559,47895301,48361861,48420271,48420919,387421219,387486109,388011061,388011709,392210029,392262589,392734981

%N Primes whose base-9 representation also is the base-2 representation of a prime.

%C This sequence is part of the two-dimensional array of sequences based on this same idea for any two different bases b, c > 1. Sequence A235265 and A235266 are the most elementary ones in this list. Sequences A089971, A089981 and A090707 through A090721, and sequences A065720 - A065727, follow the same idea with one base equal to 10.

%C For further motivation and cross-references, see sequence A235265 which is the main entry for this whole family of sequences.

%C When the smaller base is b=2 such that only digits 0 and 1 are allowed, these are primes that are the sum of distinct powers of the larger base, here c=9, thus a subsequence of A077723.

%H Zak Seidov, <a href="/A235466/b235466.txt">Table of n, a(n) for n = 1..1400</a>

%H M. F. Hasler, <a href="https://docs.google.com/document/d/10IM7fcAbB2tqRGuwfGvuEGUzD_IXbgXPDK0tfxN4M3o/pub">Primes whose base c expansion is also the base b expansion of a prime</a>

%e 739 = 1011_9 and 1011_2 = 11 are both prime, so 739 is a term.

%t fQ[n_, j_, k_] := Block[{id = IntegerDigits[n, j]}, Max[id] < k && PrimeQ[ FromDigits[ id, k]]]; lst = {}; p = 2; While[p < 4*10^9, If[ fQ[p, 9, 2], AppendTo[lst, p]; Print[p]]; p = NextPrime@ p] (* _Robert G. Wilson v_, Oct 09 2014 *)

%t pr9Q[n_]:=Module[{idn9=IntegerDigits[n,9]},Max[idn9]<2&&PrimeQ[ FromDigits[ idn9,2]]]; Select[Prime[Range[21*10^6]],pr9Q] (* _Harvey P. Dale_, Aug 25 2015 *)

%o (PARI) is(p,b=2,c=9)=vecmax(d=digits(p,c))<b&&isprime(vector(#d,i,b^(#d-i))*d~)&&isprime(p)

%o (PARI) forprime(p=1,1e3,is(p,9,2)&&print1(vector(#d=digits(p,2),i,9^(#d-i))*d~,",")) \\ To produce the terms, this is much more efficient than to select them using straightforwardly is(.)=is(.,2,9)

%Y Cf. A065720 ⊂ A036952, A065721 - A065727, A235394, A235395, A089971 ⊂ A020449, A089981, A090707 - A091924, A235461 - A235482. See the LINK for further cross-references.

%K nonn,base

%O 1,1

%A _M. F. Hasler_, Jan 11 2014