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 A235470 Primes whose base 7 representation also is the base 3 representation of a prime. 1
 2, 7, 107, 401, 443, 457, 701, 743, 751, 2417, 2753, 2843, 2851, 3089, 5147, 5153, 5503, 16823, 16921, 17207, 17257, 17551, 19553, 19993, 21617, 21673, 22003, 22303, 33623, 33679, 33721, 34301, 36017, 36373, 36457, 38873, 118057, 118343, 134507, 134857, 135151, 137251, 137593, 140057 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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. For further motivation and cross-references, see sequence A235265 which is the main entry for this whole family of sequences. Since the trailing digit of the base 7 expansion must (like all others) be less than 3, this is a subsequence of A045381. LINKS EXAMPLE E.g., 7 = 10[7] and 10[3] = 3 both are prime; 107 = 212[7] and 212[3] = 23 both are prime. PROG (PARI) is(p, b=3, c=7)=vecmax(d=digits(p, c))

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Last modified October 19 03:19 EDT 2019. Contains 328211 sequences. (Running on oeis4.)