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 A168159 Distance of the least reversible n-digit prime from 10^(n-1) 1

%I

%S 1,1,1,9,7,49,33,169,7,7,207,237,91,313,261,273,79,49,2901,51,441,193,

%T 9,531,289,1141,67,909,331,753,2613,657,49,4459,603,1531,849,2049,259,

%U 649,2119,1483,63,6747,519,3133,937,1159,1999,6921,2949,613,4137,1977,31

%N Distance of the least reversible n-digit prime from 10^(n-1)

%C A (much) more compact form of A114018 (cf. formula). Since this sequence and A114018 refer to "reversible primes" (A007500), while A122490 seems to use "emirps" (A006567), a(n+1) differs from A122490(n) iff 10^n+1 is prime <=> a(n+1)=1 <=> A114018(n)=10^n+1.

%F a(n)=A114018(n)-10^(n-1)

%t Table[p = NextPrime[y = 10^(n - 1)]; While[! PrimeQ[FromDigits[Reverse[IntegerDigits[p]]]], p = NextPrime[p]]; p - y, {n, 55}] (* _Jayanta Basu_, Aug 09 2013 *)

%o (PARI) for(x=1,1e99, until( isprime(x=nextprime(x+1)) & isprime(eval(concat(vecextract(Vec(Str(x)),"-1..1")))),);print1(x-10^ (#Str(x)-1),", "); x=10^#Str(x)-1)

%K base,nonn

%O 1,4

%A _M. F. Hasler_, Nov 21 2009

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Last modified July 22 08:31 EDT 2019. Contains 325216 sequences. (Running on oeis4.)