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 A075244 Least number requiring the base n to produce a prime by base reversal. 0
 2, 3, 15, 8, 109, 9, 119, 16, 27, 70, 2197, 36, 1265, 158, 213, 178, 4205, 126, 14189, 260, 273, 304, 4865, 120, 1295, 78, 81, 532, 44323, 150, 47317, 952, 771, 102, 16705, 492, 6209, 114, 1209, 2020, 132743, 294, 22945, 2834, 2721, 2276, 66455, 144 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Question, Is every base necessary to convert the natural numbers into primes? LINKS EXAMPLE a(1) = 2 because two = 11 in unary (A000042) and its reversal 11 = 2. a(2) = 3 because three = 11 in base 2 (A007088) and its reversal 11 in base 2 = 3. a(3) = 15 because fifteen = 120 in base 3 (A007089) and its reversal 21 in base 3 = 7. a(4) = 8 -> 2. a(7) = 119 because 119 base 7 = 230 in base 7 (A007093) and its reversal 32 base 7 = 161. MATHEMATICA f[n_] := Block[{b = 2}, While[b < n && !PrimeQ[ FromDigits[ Reverse[ IntegerDigits[n, b]], b]], b++ ]; If[b != n, b, 0]]; a = Table[0, {70}]; Do[b = f[n]; If[b < 76 && a[[b]] == 0, a[[b]] = n], {n, 2, 133000}] CROSSREFS Cf. A075241 & A075242. Sequence in context: A180698 A266618 A082572 * A342567 A088030 A101047 Adjacent sequences:  A075241 A075242 A075243 * A075245 A075246 A075247 KEYWORD base,easy,nonn AUTHOR Robert G. Wilson v, Sep 09 2002 STATUS approved

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Last modified September 25 11:03 EDT 2021. Contains 347654 sequences. (Running on oeis4.)