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A075244
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Least number requiring the base n to produce a prime by base reversal.
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0
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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; internal format)
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OFFSET
| 1,1
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COMMENTS
| Question, Is every base necessary to convert the natural numbers into primes?
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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.
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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}]
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CROSSREFS
| Cf. A075241 & A075242.
Sequence in context: A042817 A180698 A082572 * A088030 A101047 A066491
Adjacent sequences: A075241 A075242 A075243 * A075245 A075246 A075247
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KEYWORD
| base,easy,nonn
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AUTHOR
| Robert G. Wilson v (rgwv(AT)rgwv.com), Sep 09 2002
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