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A003684
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Number of n-digit reversible primes (or emirps) with distinct digits.
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21
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OFFSET
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1,1
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LINKS
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EXAMPLE
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13, 17, 31, 37, 71, 73, 79 and 97 are reversible primes (emirps), so a(2)=8.
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MATHEMATICA
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emrpQ[n_]:=Module[{idn=IntegerDigits[n], rev}, rev=Reverse[idn]; rev!=idn && Max[DigitCount[n]] ==1&&PrimeQ[FromDigits[rev]]]; With[{ems=Select[ Prime[ Range[ 51*10^6]], emrpQ]}, Join[ {4}, Table[Count[ems, _?(IntegerLength[ #] == n&)], {n, 2, 9}]]] (* Harvey P. Dale, Nov 29 2014 *)
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PROG
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(Python)
from sympy import primerange, isprime
return len([p for p in primerange(10**(n-1), 10**n)
if len(set(str(p))) == len(str(p)) and isprime(int(str(p)[::-1]))])
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CROSSREFS
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KEYWORD
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nonn,fini,full,base,easy,nice
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AUTHOR
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EXTENSIONS
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Typo in example corrected by David Ritterskamp (dritters(AT)usi.edu), Mar 24 2008
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STATUS
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approved
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