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A108505 Number of palindromic semiprimes less than 10^n. 1
0, 3, 7, 36, 50, 269, 367, 2181, 2816, 18391, 23617, 160773, 203733, 1429749, 1788486, 12808711, 15889727 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Does the limit n-> inf. a(n+2)/a(n) =~ 8*Pi^2/9?

LINKS

Table of n, a(n) for n=0..16.

MATHEMATICA

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]]]] ]]]]; fQ[n_] := Plus @@ Last /@ FactorInteger[n] == 2; c = np = 0; Do[ While[np < 10^n, If[ fQ[np], c++ ]; np = NextPalindrome[np]]; Print[c], {n, 0, 12}]

CROSSREFS

Cf. A046328.

Sequence in context: A132102 A081555 A063042 * A047158 A221511 A102917

Adjacent sequences:  A108502 A108503 A108504 * A108506 A108507 A108508

KEYWORD

base,nonn

AUTHOR

Robert G. Wilson v, Jun 06 2005

EXTENSIONS

a(15)-a(16) from Donovan Johnson, Mar 14 2010

STATUS

approved

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Last modified August 18 16:01 EDT 2017. Contains 290727 sequences.