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A373765 - OEIS
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A373765 a(n) is the first number that starts a sequence of exactly n primes x(1), ..., x(n) where x(i+1) = A004093(x(i)) is the digit reversal of 2 * x(i). 0
2, 7, 19, 487, 1637, 389047 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
a(7) > 7 * 10^9 if it exists.
LINKS
EXAMPLE
a(1) = 2 because 2 is prime while A004093(2) = 4 is not prime.
a(2) = 7 because 7 and A004093(7) = 41 are prime but A004093(41) = 28 is not.
a(3) = 19 because 19 and A004093(19) = 83 and A004093(83) = 661 are prime but A004093(661) = 2231 is not.
MAPLE
g:= proc(n) local t, L, i, j;
t:= n;
for i from 0 while isprime(t) do
L:= convert(2*t, base, 10);
t:= add(L[-j]*10^(j-1), j=1..nops(L));
od;
i
end proc:
V:= Vector(6): count:= 0: p:= 1:
while count < 6 do
p:= nextprime(p); v:= g(p);
if V[v] = 0 then V[v]:= p; count:= count+1 fi
od:
convert(V, list);
CROSSREFS
Cf. A004093.
Sequence in context: A137990 A056650 A182169 * A170869 A162355 A045858
KEYWORD
nonn,base,more
AUTHOR
Robert Israel, Jun 18 2024
STATUS
approved

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Last modified August 13 12:37 EDT 2024. Contains 375141 sequences. (Running on oeis4.)