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Search: id:A090744
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| A090744 |
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Consider numbers of the form ...53197531975319753, whose digits read from the right are 3,5,7,9,1,3,5,7,9,1,3,... Sequence gives lengths of these numbers which are primes. |
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2
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OFFSET
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0,2
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EXAMPLE
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a(1)=1 because 3 is prime and 3 has 1 digit.
a(2)=2 because 53 is prime and 53 has 2 digits
a(3)=5 because 19753 is prime and 19753 has 5 digits
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MATHEMATICA
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s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 4, 1, 0, 3, 1, 5, 2, 7, 3, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 3000}] (from Robert G. Wilson v Feb 19 2004)
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PROGRAM
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(PARI) test3(n)= s=0; for(i=0, n, \ if(Mod(i, 5)==0, s=s+3*10^i, \ if(Mod(i, 5)==1, s=s+5*10^i, \ if(Mod(i, 5)==2, s=s+7*10^i, \ if(Mod(i, 5)==3, s=s+9*10^i, \ if(Mod(i, 5)==4, s=s+1*10^i, \ )))))); return(s); for(j=0, 1000, if(isprime(test3(j)), print(j+1), print1(".")))
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CROSSREFS
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Sequence in context: A003095 A023362 A138613 this_sequence A041571 A042259 A100105
Adjacent sequences: A090741 A090742 A090743 this_sequence A090745 A090746 A090747
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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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1932 from Robert G. Wilson v (rgwv(AT)rgwv.com), Feb 10 2004
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