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A367796
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Primes p such that the sum of p and its reversal is the square of a prime.
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4
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2, 29, 47, 83, 20147, 23117, 24107, 63113, 80141, 81131, 261104399, 262005299, 262104299, 262203299, 263302199, 264203099, 264302099, 264500099, 270401489, 271500389, 273104189, 273302189, 274401089, 282203279, 284302079, 284500079, 291104369, 291203369, 292005269, 293005169, 293104169, 294302069
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OFFSET
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1,1
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COMMENTS
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Terms > 83 have an odd number of digits and an even first digit.
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LINKS
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EXAMPLE
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A056964(a(n)) = 121 = 11^2 for 2 <= n <= 4.
A056964(a(n)) = 94249 = 307^2 for 5 <= n <= 10.
A056964(a(n)) = 1254505561 = 35419^2 for 11 <= n <= 71.
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MAPLE
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digrev:= proc(n) local L, i;
L:= convert(n, base, 10);
add(L[-i]*10^(i-1), i=1..nops(L))
end proc:
filter:= proc(t) local v;
v:= sqrt(t+digrev(t));
v::integer and isprime(v)
end proc:
R:= 2, 29, 47, 83: count:= 4: flag:= true:
for d from 3 to 9 by 2 do
p:= prevprime(10^(d-1));
for i from 1 do
p:= nextprime(p);
p1:= floor(p/10^(d-1));
if p1::odd then p:= nextprime((p1+1)*10^(d-1)) fi;
if p > 10^d then break fi;
if filter(p) then
count:= count+1; R:= R, p;
fi od od:
R;
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MATHEMATICA
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Select[Prime[Range[10^6]], PrimeQ[Sqrt[#+FromDigits[Reverse[IntegerDigits[#]]]]] &] (* Stefano Spezia, Dec 10 2023 *)
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PROG
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(PARI) \\ See PARI link
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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