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Primes in A038395 (concatenate first n odd numbers in reverse order).
1

%I #15 Apr 15 2024 05:00:39

%S 31,

%T 737169676563615957555351494745434139373533312927252321191715131197531,

%U 12312111911711511311110910710510310199979593918987858381797775737169676563615957555351494745434139373533312927252321191715131197531

%N Primes in A038395 (concatenate first n odd numbers in reverse order).

%C Among the first 72 terms only three terms are prime SmBackConOdd(2), SmBackConOdd(37) and SmBackConOdd(62), found by Micha Fleuren.

%C The next term (a(4)) has 1172 digits. - _Harvey P. Dale_, Jul 12 2022

%C a(5) = A038395(2717) has 10313 digits. - _Robert Israel_, Apr 14 2024

%H M. Fleuren, <a href="http://www.gallup.unm.edu/~smarandache/SmBackConOdd.txt">Smarandache Back Concatenated Odds</a>.

%H Mathematics StackExchange, <a href="https://math.stackexchange.com/questions/4898990/is-it-known-whether-or-not-oeis-sequence-a091308-is-finite">Is it known whether or not OEIS sequence A091308 is finite?</a>

%p x:= 1: d:= 1: count:= 0: R:= NULL:

%p for i from 2 while count < 5 do

%p x:= (2*i-1)*10^d + x;

%p d:= d + ilog10(2*i-1)+1;

%p if isprime(x) then count:= count+1; R:= R,x; fi;

%p od:

%p R; # _Robert Israel_, Apr 14 2024

%t Select[Table[FromDigits[Flatten[IntegerDigits/@Range[n,1,-2]]],{n,1,341,2}],PrimeQ] (* _Harvey P. Dale_, Jul 12 2022 *)

%Y Cf. A038395.

%K nonn,bref,base

%O 1,1

%A M. Grete (martha.grete(AT)web.de), Feb 21 2004

%E Definition corrected by _XU Pingya_, Mar 16 2017