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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 that are primes. 2
 2, 8, 11, 21, 54, 734, 5636, 14954 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Table of n, a(n) for n=1..8. 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}] (* 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)), print1(j+1, ", "))) CROSSREFS Cf. A090743, A090744, A090745. Sequence in context: A295070 A009420 A189328 * A362869 A234924 A336771 Adjacent sequences: A090743 A090744 A090745 * A090747 A090748 A090749 KEYWORD hard,nonn,base,more,less AUTHOR mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004 EXTENSIONS a(6) from Robert G. Wilson v, Feb 10 2004 a(7) from Donovan Johnson, Mar 06 2008 Offset changed to 1 and a(8) from Michael S. Branicky, May 27 2023 STATUS approved

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Last modified February 28 07:05 EST 2024. Contains 370387 sequences. (Running on oeis4.)