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A086697 Left-truncatable semiprimes, i.e., semiprimes in which repeatedly deleting the leftmost digit gives a semiprime at every step until a single-digit semiprime remains. 1
4, 6, 9, 14, 26, 34, 39, 46, 49, 69, 74, 86, 94, 134, 146, 169, 194, 214, 226, 249, 274, 314, 326, 334, 339, 346, 386, 394, 446, 469, 514, 526, 586, 614, 626, 634, 649, 669, 674, 694, 734, 746, 749, 794, 849, 869, 886, 914, 926, 934, 939, 949, 974, 1169, 1214 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Zero digits are not permitted, so 106 is not a member even though 106 and 6 are both semiprimes. - Harvey P. Dale, Jun 28 2017

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

I. O. Angell and H. J. Godwin, On Truncatable Primes, Math. Comput. 31, 265-267, 1977.

Index entries for sequences related to truncatable primes

EXAMPLE

a(15)=146 is a term because 146, 46, 6 are all semiprimes.

MATHEMATICA

ltsQ[n_]:=DigitCount[n, 10, 0]==0&&AllTrue[FromDigits/@NestList[Rest[ #]&, IntegerDigits[n], IntegerLength[n]-1], PrimeOmega[#]==2&]; Select[ Range[ 1500], ltsQ] (* Harvey P. Dale, Jun 28 2017 *)

lt3pQ[n_]:=Module[{idn=IntegerDigits[n]}, FreeQ[idn, 0]&&Union[PrimeOmega/@(FromDigits/@Table[Take[idn, -i], {i, Length[idn]}])]=={2}]; Select[Range[8000], lt3pQ] (* Vincenzo Librandi, Apr 22 2018 *)

CROSSREFS

Cf. A001358 (semiprimes).

Sequence in context: A226271 A137371 A179463 * A287568 A251728 A078443

Adjacent sequences:  A086694 A086695 A086696 * A086698 A086699 A086700

KEYWORD

base,nonn

AUTHOR

Shyam Sunder Gupta, Jul 28 2003

STATUS

approved

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Last modified February 15 22:28 EST 2019. Contains 320138 sequences. (Running on oeis4.)