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A046941
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Palindromic primes whose indices n are also palindromes.
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6
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OFFSET
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1,1
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LINKS
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Chris K. Caldwell and G. L. Honaker, Jr., 143787341
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FORMULA
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MATHEMATICA
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NextPalindrome[n_] := Block[ {l = Floor[ Log[10, n] + 1], idn = IntegerDigits[n]}, If[ Union[ idn] == {9}, Return[n + 2], If[l < 2, Return[n + 1], If[ FromDigits[ Reverse[ Take[ idn, Ceiling[l/2]]]] FromDigits[ Take[ idn, -Ceiling[l/2]]], FromDigits[ Join[ Take[ idn, Ceiling[l/2]], Reverse[ Take[ idn, Floor[l/2]]] ]], idfhn = FromDigits[ Take[ idn, Ceiling[l/2]]] + 1; idp = FromDigits[ Join[ IntegerDigits[ idfhn], Drop[ Reverse[ IntegerDigits[ idfhn]], Mod[l, 2]]]] ]]]];
p = 0; Do[p = NextPalindrome[p]; While[ !PrimeQ[p], p = NextPalindrome[ p]]; q = IntegerDigits[ PrimePi[ p]]; If[Reverse[q] == q, Print[{p, FromDigits[q]}]], {n, 10^4}] (* Robert G. Wilson v, Feb 03 2005 *)
palQ[n_] := Reverse[x = IntegerDigits[n]] == x; t = {}; Do[p = Prime[i]; If[palQ[i] && palQ[p], AppendTo[t, p]], {i, 9*10^6}]; t (* Jayanta Basu, Jun 23 2013 *)
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PROG
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(PARI) ispal(n) = my(d=digits(n)); d == Vecrev(d);
isok(p) = isprime(p) && ispal(p) && ispal(primepi(p)); \\ Michel Marcus, Jan 27 2019
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CROSSREFS
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KEYWORD
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nonn,base,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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