A358983 - OEIS

A358983

a(n) is the first emirp p that starts a sequence of n emirps x(1),...,x(n) with x(1) = p and x(k+1) = 2*x(k) - reverse(x(k)), but 2*x(n) - reverse(x(n)) is not an emirp.

13, 941, 1471, 120511, 368631127

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

OFFSET

1,1

LINKS

Table of n, a(n) for n=1..5.

EXAMPLE

a(3) = 1471 because 1471 is an emirp, 2*1471 - 1741 = 1201 is an emirp, and 2*1201 - 1021 = 1381 is an emirp, but 2*1381 - 1831 = 931 is not an emirp.

MAPLE

rev:= proc(n) local L, t;

L:= convert(n, base, 10);

add(L[-t]*10^(t-1), t=1..nops(L));

end proc:

g:= proc(n) local x, n, r;

t:= 0; x:= n;

r:= rev(x);

if r = x or not isprime(x) or not isprime(r) then return t fi;

t:= t+1;

x:= 2*x - r;

end proc:

V:= Vector(5): count:= 0:

for n from 13 by 2 while count < 5 do

v:= g(n);

if v > 0 and V[v] = 0 then

V[v]:= n; count:= count+1

od:

convert(V, list);

CROSSREFS

Cf. A006567, A358689.

Sequence in context: A274544 A376463 A267915 * A096084 A203708 A342811

Adjacent sequences: A358980 A358981 A358982 * A358984 A358985 A358986

KEYWORD

nonn,base,more

AUTHOR

J. M. Bergot and Robert Israel, Dec 08 2022

STATUS

approved