A173916 Primes of the form R(n!!-1), where R(k) is the digit reversal of k.

2, 7, 41, 383, 401, 449, 9911985173, 422341432613, 9991380634172583438761, 426098774309076039404423265983175582575572211, 4260466555914046005704099160490768769763030425914059033564091456450512175875462052616893583

Offset

1,1

Links

Table of n, a(n) for n=1..11.

Example

n=7 -> 7!!-1 = 105-1=104 -> R(104)=401 is prime

Maple

P:=proc(i) local a, b, k, n, v; v:=array(1..10000); for n from 1 by 1 to i do a:=1; k:=doublefactorial(n)-1; while k>0 do v[a]:=k-(trunc(k/10)*10); k:=trunc(k/10); a:=a+1; od; k:=0; for b from a-1 by -1 to 1 do k:=k+v[b]*10^(a-1-b); od; if isprime(k) then print(k); fi; od; end: P(2000);

Crossrefs

Cf. A173913-A173915

Sequence in context: A320415 A006846 A047864 * A163921 A213434 A331920

Adjacent sequences: A173913 A173914 A173915 * A173917 A173918 A173919

Keyword

nonn,base

Author

Paolo P. Lava and Giorgio Balzarotti, Mar 02 2010, Mar 05 2010

Extensions

Edited by Charles R Greathouse IV, Aug 02 2010

Status

approved