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A189484 Numbers that can be factored into semiprimes which, when concatenated in increasing order, produce a palindrome. 0
1, 4, 6, 9, 16, 22, 33, 36, 55, 56, 64, 77, 81, 111, 121, 136, 141, 156, 161, 202, 216, 256, 262, 276, 296, 303, 323, 351, 376, 393, 441, 454, 484, 505, 515, 516, 535, 545, 560, 565, 621, 626, 707, 717, 729, 737, 765, 767, 784, 818, 838, 878, 898, 939, 949 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

This is to semiprimes A001358 as A046447 is to primes A000040.

The initial 1 represents the empty product.

LINKS

Table of n, a(n) for n=1..55.

EXAMPLE

The first value not itself a semiprime palindrome (A046328) or power of semiprimes (i.e., 16 = 4 * 4 which concatenate to the palindrome 44, 484 = 22^2) is 56 = 4 * 14. The first where additionally the first factor is not a single digit is 765 = 15 * 51 = 3^2 * 5 * 17 since (15, 51) are a pair of emirpimes A097393, and 765 = A158126(1).

MATHEMATICA

ok[n_] := n == 1 || Block[{d, p = Join @@ mu /@ FactorInteger[n]}, EvenQ@ Length[p] && AnyTrue[ Union[ Sort /@ ((Times @@@ #) & /@ Union[ (Sort /@ Partition[#, 2]) & /@ Permutations[p]])], (d = Join @@ IntegerDigits[#]; d == Reverse[d]) &]]; Select[ Range[1000], ok] (* Giovanni Resta, Sep 15 2018 *)

CROSSREFS

Cf. A001358, A002113, A046328, A046447, A097393, A158126 Products of emirpimes pairs, sorted.

Sequence in context: A266346 A181018 A155569 * A155567 A225377 A036667

Adjacent sequences:  A189481 A189482 A189483 * A189485 A189486 A189487

KEYWORD

nonn,base

AUTHOR

Jonathan Vos Post, Apr 22 2011

EXTENSIONS

Additional terms from Franklin T. Adams-Watters, Apr 28 2011

More terms from Giovanni Resta, Sep 15 2018

STATUS

approved

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Last modified August 13 00:27 EDT 2020. Contains 336441 sequences. (Running on oeis4.)