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 A090746 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. 1
 2, 8, 11, 21, 54, 734, 5636 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 LINKS EXAMPLE a(1)=2 because 19 is prime and 19 has 2 digits a(2)=8 because 31975319 is prime and 31975319 has 8 digits MATHEMATICA 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) PROG (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("."))) CROSSREFS Sequence in context: A074263 A009420 A189328 * A174114 A197540 A089118 Adjacent sequences:  A090743 A090744 A090745 * A090747 A090748 A090749 KEYWORD hard,nonn,base,more AUTHOR mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004 EXTENSIONS 734 from Robert G. Wilson v, Feb 10 2004 a(6) from Donovan Johnson, Mar 06 2008 STATUS approved

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