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Search: id:A090743
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| A090743 |
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Consider numbers of the form ...31975319753197531, whose digits read from the right are 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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1,1
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EXAMPLE
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1 is not a prime; 31 is a 2-digit prime, so 2 is a member.
531 is not prime, 7531 is not prime, 97531 is not prime, 197531 is not prime.
a(2)=58 because 5319753197531975319753197531975319753197531975319753197531 is prime and has 58 digits
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MATHEMATICA
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s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 0, 1, 1, 3, 2, 5, 3, 7, 4, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 2500}] (from Robert G. Wilson v Feb 19 2004)
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PROGRAM
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(PARI) test1(n)= s=0; for(i=0, n, \ if(Mod(i, 5)==0, s=s+1*10^i, \ if(Mod(i, 5)==1, s=s+3*10^i, \ if(Mod(i, 5)==2, s=s+5*10^i, \ if(Mod(i, 5)==3, s=s+7*10^i, \ if(Mod(i, 5)==4, s=s+9*10^i, \ )))))); return(s); for(j=0, 2000, if(isprime(test1(j)), print(j+1), print1(".")))
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CROSSREFS
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Cf. A090744, A090745, A090746.
Sequence in context: A058196 A024237 A030263 this_sequence A104494 A121931 A115610
Adjacent sequences: A090740 A090741 A090742 this_sequence A090744 A090745 A090746
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KEYWORD
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hard,nonn,base
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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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Robert G. Wilson v reports no further terms below 5456. Feb 18 2004.
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