A068640 - OEIS

A068640

Define f(n) = 2n+1, a(n) = largest prime of the form f(f(f(...(n))). If no such prime exists then a(n) = 0.

7, 47, 7, 0, 47, 13, 0, 17, 19, 0, 47, 0, 0, 59, 31, 0, 0, 37, 0, 167, 43, 0, 47, 0, 0, 107, 0, 0, 59, 61, 0, 0, 67, 0, 71, 73, 0, 0, 79, 0, 167, 0, 0, 2879, 0, 0, 0, 97, 0, 101, 103, 0, 107, 109, 0, 227, 0, 0, 0, 0, 0, 0, 127, 0, 263, 0, 0, 137, 139, 0, 0, 0, 0, 149, 151, 0, 0, 157, 0

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Table of n, a(n) for n=1..79.

EXAMPLE

a(2) = 47 as f(2) = 5, f(5) = 11,f(11) = 23, f(23) = 47 is the largest such prime . f(47) = 95 is not a prime. a(4) = 0 as f(4) = 9 is composite.

MAPLE

for k from 1 to 500 do a := 2*k+1; while(isprime(a)) do a := 2*a+1; end do; c[k] := (a-1)/2; if(not isprime(c[k])) then c[k] := 0; end if; if(c[k]<2*k+1) then c[k] := 0; end if; end do:q := seq(c[i], i=1..500);

CROSSREFS

Cf. A068638.

Sequence in context: A197754 A000823 A036944 * A089725 A086040 A009241

Adjacent sequences: A068637 A068638 A068639 * A068641 A068642 A068643

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Feb 27 2002

EXTENSIONS

More terms from Sascha Kurz, Mar 17 2002

STATUS

approved