A091308 Primes in A038395 (concatenate first n odd numbers in reverse order).

31, 737169676563615957555351494745434139373533312927252321191715131197531, 12312111911711511311110910710510310199979593918987858381797775737169676563615957555351494745434139373533312927252321191715131197531

Offset: 1

Author

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

Keywords

• nonn
• bref
• base

Comments

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

The next term (a(4)) has 1172 digits. - Harvey P. Dale, Jul 12 2022

a(5) = A038395(2717) has 10313 digits. - Robert Israel, Apr 14 2024

Links

• M. Fleuren, Smarandache Back Concatenated Odds.
• Mathematics StackExchange, Is it known whether or not OEIS sequence A091308 is finite?

Crossrefs

Cf. A038395.

Programs

Maple

x:= 1: d:= 1: count:= 0: R:= NULL:
for i from 2 while count < 5 do
  x:= (2*i-1)*10^d + x;
  d:= d + ilog10(2*i-1)+1;
  if isprime(x) then count:= count+1; R:= R,x; fi;
od:
R; # Robert Israel, Apr 14 2024

Mathematica

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

Extensions

Definition corrected by XU Pingya, Mar 16 2017