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A090745
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Consider numbers of the form ...7531975319753197, whose digits read from the right are 7,9,1,3,5,7,9,1,3,5,7,... Sequence gives lengths of these numbers which are primes.
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1
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1, 2, 3, 5, 6, 7, 16, 17, 18, 104, 127, 213, 1143
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,2
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EXAMPLE
| a(1)=1 because 7 is prime and 7 has 1 digits
a(2)=2 because 97 is prime and 97 has 2 digits
a(3)=3 because 197 is prime and 197 has 3 digits
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MATHEMATICA
| s = 0; Do[s = s + 10^n*Switch[ Mod[n, 5], 2, 1, 3, 3, 4, 5, 0, 7, 1, 9]; If[ PrimeQ[s], Print[n + 1]], {n, 0, 3000}] (from Robert G. Wilson v Feb 10 2004)
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PROG
| (PARI) test7(n)= s=0; for(i=0, n, \ if(Mod(i, 5)==0, s=s+3*10^i, \ if(Mod(i, 5)==1, s=s+7*10^i, \ if(Mod(i, 5)==2, s=s+9*10^i, \ if(Mod(i, 5)==3, s=s+1*10^i, \ if(Mod(i, 5)==4, s=s+5*10^i, \ )))))); return(s); for(j=0, 1000, if(isprime(test7(j)), print(j+1), print1(".")))
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CROSSREFS
| Sequence in context: A103538 A144671 A073721 * A002229 A146747 A077674
Adjacent sequences: A090742 A090743 A090744 * A090746 A090747 A090748
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KEYWORD
| hard,nonn,base,more
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AUTHOR
| mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004
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EXTENSIONS
| Corrected and extended by Robert G. Wilson v (rgwv(AT)rgwv.com), Feb 10 2004
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