A231479 -id:A231479 - cp's OEIS frontend

A235621 Primes whose base-9 representation also is the base-7 representation of a prime.

2, 3, 5, 13, 23, 29, 37, 47, 59, 103, 109, 131, 167, 173, 181, 199, 211, 263, 283, 379, 419, 509, 541, 733, 787, 821, 859, 911, 919, 983, 1013, 1063, 1091, 1093, 1171, 1487, 1499, 1543, 1549, 1559, 1567, 1571, 1667, 1669, 1733, 1783, 1787, 1913, 1993, 2237, 2287, 2351, 2381, 2477, 2621

Offset: 1

M. F. Hasler, Jan 13 2014

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.

			E.g., 13 = 14_9 and 14_7 = 11 are both prime.

Robert Price, Table of n, a(n) for n = 1..14672
M. F. Hasler, Primes whose base c expansion is also the base b expansion of a prime

Cf. A231479, A235265, A235266, A152079, A235461 - A235482, A065720 - A065727, A235394, A235395, A089971 ⊂ A020449, A089981, A090707 - A091924, A235615 - A235639. See the LINK for further cross-references.

is(p,b=7,c=9)=vecmax(d=digits(p,c))

forprime(p=1,3e3,is(p,9,7)&&print1(vector(#d=digits(p,7),i,9^(#d-i))*d~,",")) \\ To produce the terms, this is more efficient than to select them using straightforwardly is(.)=is(.,7,9)

cp's OEIS Frontend

A235621 Primes whose base-9 representation also is the base-7 representation of a prime.

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