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A235634 Primes whose base-4 representation is also the base-7 representation of a prime. 2
2, 3, 11, 23, 29, 31, 41, 71, 79, 101, 109, 113, 137, 149, 157, 163, 191, 199, 239, 251, 263, 269, 283, 307, 353, 397, 401, 431, 443, 521, 547, 601, 701, 709, 743, 751, 773, 853, 887, 941, 947, 983, 1013, 1039, 1049, 1069, 1109, 1151, 1217, 1283, 1303, 1487, 1489, 1663, 1669, 1789, 1823, 1901, 1949, 1973 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This sequence is part of a two dimensional array of sequences, given in the LINK, 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.

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

M. F. Hasler, Primes whose base c expansion is also the base b expansion of a prime

EXAMPLE

E.g., 11 = 23_4 and 23_7 = 17 both are prime.

MAPLE

filter:= proc(n) local L, m;

  if not isprime(n) then return false fi;

  L:= convert(n, base, 4);

  isprime(add(L[i]*7^(i-1), i=1..nops(L)));

end proc:

select(filter, [2, seq(i, i=3..10000, 2)]); # Robert Israel, Jul 02 2018

PROG

(PARI) is(p, b=7, c=4)=isprime(vector(#d=digits(p, c), i, b^(#d-i))*d~)&&isprime(p) \\ Note: This code is only valid for b > c.

CROSSREFS

Cf. A235617, A235265, A235266, A152079, A235461 - A235482, A065720 - A065727, A235394, A235395, A089971, A020449, A089981, A090707 - A091924, A235615 - A235639. See the LINK for further cross-references.

Sequence in context: A229805 A104075 A236168 * A070174 A320393 A080153

Adjacent sequences:  A235631 A235632 A235633 * A235635 A235636 A235637

KEYWORD

nonn,base

AUTHOR

M. F. Hasler, Jan 13 2014

STATUS

approved

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Last modified April 5 13:33 EDT 2020. Contains 333241 sequences. (Running on oeis4.)