

A231479


Primes whose base7 representation is also the base9 representation of a prime.


2



2, 3, 5, 11, 19, 23, 29, 37, 47, 67, 71, 89, 103, 107, 113, 127, 137, 163, 179, 239, 257, 313, 337, 347, 389, 401, 431, 457, 463, 499, 523, 547, 569, 571, 617, 709, 719, 739, 743, 751, 757, 761, 821, 823, 859, 883, 887, 971, 1019, 1069, 1093, 1129, 1153, 1213, 1297, 1307, 1327, 1367, 1373, 1381
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OFFSET

1,1


COMMENTS

This sequence is part of a twodimensional 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

Giovanni Resta, 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

11 = 14_7 and 14_9 = 13 are both prime, so 11 is a term.


MATHEMATICA

Select[Prime@ Range@ 250, PrimeQ@ FromDigits[IntegerDigits[#, 7], 9] &] (* Giovanni Resta, Sep 12 2019 *)


PROG

(PARI) is(p, b=9, c=7)=isprime(vector(#d=digits(p, c), i, b^(#di))*d~)&&isprime(p) \\ Note: This code is only valid for b > c.


CROSSREFS

Cf. A235621, A235265, A235266, A152079, A235461  A235482, A065720 ⊂ A036952, A065721  A065727, A235394, A235395, A089971 ⊂ A020449, A089981, A090707  A091924. See the LINK for further crossreferences.
Sequence in context: A025067 A024371 A344963 * A084758 A087582 A235661
Adjacent sequences: A231476 A231477 A231478 * A231480 A231481 A231482


KEYWORD

nonn,base


AUTHOR

M. F. Hasler, Jan 12 2014


STATUS

approved



