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A090746
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Consider numbers of the form ...19753197531975319, whose digits read from the right are 9,1,3,5,7,9,1,3,5,7,9,1,... Sequence gives lengths of these numbers which are primes.
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1
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OFFSET
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0,1
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LINKS
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Table of n, a(n) for n=0..6.
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EXAMPLE
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a(1)=2 because 19 is prime and 19 has 2 digits
a(2)=8 because 31975319 is prime and 31975319 has 8 digits
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MATHEMATICA
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s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 1, 1, 2, 3, 3, 5, 4, 7, 0, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 3000}] (from Robert G. Wilson v Feb 10 2004)
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PROG
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(PARI) test9(n)= s=0; for(i=0, n, \ if(Mod(i, 5)==0, s=s+9*10^i, \ if(Mod(i, 5)==1, s=s+1*10^i, \ if(Mod(i, 5)==2, s=s+3*10^i, \ if(Mod(i, 5)==3, s=s+5*10^i, \ if(Mod(i, 5)==4, s=s+7*10^i, \ )))))); return(s); for(j=0, 732, if(isprime(test9(j)), print(j+1), print1(".")))
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CROSSREFS
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Sequence in context: A074263 A009420 A189328 * A174114 A197540 A089118
Adjacent sequences: A090743 A090744 A090745 * A090747 A090748 A090749
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KEYWORD
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hard,nonn,base,more
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AUTHOR
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mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004
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EXTENSIONS
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734 from Robert G. Wilson v, Feb 10 2004
a(6) from Donovan Johnson, Mar 06 2008
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STATUS
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approved
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