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 A090745 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 that are primes. 2
 1, 2, 3, 5, 6, 7, 16, 17, 18, 104, 127, 213, 1143, 4654, 4897, 17046, 33424 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that 53197 * (10^(5*m)-1)/99999 + (53197 mod 10^(n mod 5))*10^(5*m) is prime, where m = floor(n/5). - Max Alekseyev, Nov 11 2012 LINKS Table of n, a(n) for n=1..17. 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. 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}] (* Robert G. Wilson v, Feb 10 2004 *) Position[Table[FromDigits[PadLeft[{}, n, {5, 3, 1, 9, 7}]], {n, 1200}], _?PrimeQ]// Flatten (* Harvey P. Dale, Mar 30 2019 *) PROG (PARI) test7(n)= s=0; for(i=0, n, if(Mod(i, 5)==0, s=s+7*10^i, if(Mod(i, 5)==1, s=s+9*10^i, if(Mod(i, 5)==2, s=s+1*10^i, if(Mod(i, 5)==3, s=s+3*10^i, if(Mod(i, 5)==4, s=s+5*10^i, )))))); return(s); for(j=0, 1000, if(isprime(test7(j)), print1(j+1, ", "))) \\ corrected by Michael S. Branicky and Michel Marcus, May 28 2023 CROSSREFS Cf. A090743, A090744, A090746. Sequence in context: A144671 A073721 A285639 * A002229 A299158 A146747 Adjacent sequences: A090742 A090743 A090744 * A090746 A090747 A090748 KEYWORD hard,nonn,base,more,less AUTHOR mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 03 2004 EXTENSIONS Corrected and extended by Robert G. Wilson v, Feb 10 2004 Offset changed to 1 and a(14)-a(17) from Michael S. Branicky, May 28 2023 STATUS approved

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Last modified December 8 03:15 EST 2023. Contains 367662 sequences. (Running on oeis4.)