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 A085733 Right-truncatable semiprimes. 6
 4, 6, 9, 46, 49, 62, 65, 69, 91, 93, 94, 95, 466, 469, 493, 497, 622, 623, 626, 629, 655, 694, 695, 697, 698, 699, 913, 914, 917, 933, 934, 939, 943, 949, 951, 955, 958, 959, 4661, 4666, 4667, 4694, 4699, 4934, 4939, 4971, 4979, 6227, 6233, 6238 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Semiprimes in which repeatedly deleting the rightmost digit gives a semiprime at every step until a single-digit semiprime remains. The sequence is finite. According to Shyam Sunder Gupta the number 95861957783594714393831931415189937897 is the largest right-truncatable semiprime. The total number of right-truncatable semiprimes including the single-digit semiprimes 4, 6 and 9 is 56076. - Shyam Sunder Gupta, Jan 13 2008 No term ends in (or contains) 0 else it would be divisible by 2, 5, and some other factor. - Michael S. Branicky, Aug 04 2022 REFERENCES Shyam Sunder Gupta, Truncatable semi-primes, Mathematical Spectrum 39:3 (2007), pp. 109-112. LINKS Michael S. Branicky, Table of n, a(n) for n = 1..56076 (full sequence). I. O. Angell and H. J. Godwin, On truncatable primes, Math. Comput. 31:137, 265-267, 1977. G. L. Honaker, Jr., Prime Curios! 46613113151331733179 Shyam Sunder Gupta, The largest right-truncatable semiprime. Prime Curios. Index entries for sequences related to truncatable primes PROG (Python) from sympy import factorint from itertools import islice def issemiprime(n): return sum(factorint(n).values()) == 2 def agen(): semis, digits = [4, 6, 9], "123456789" # can't end in 0 while len(semis) > 0: yield from semis cands = set(int(str(p)+d) for p in semis for d in digits) semis = sorted(c for c in cands if issemiprime(c)) print(list(islice(agen(), 50))) # Michael S. Branicky, Aug 04 2022 CROSSREFS Cf. A001358, A213019, A086697. Sequence in context: A175459 A257652 A107665 * A242751 A107342 A086698 Adjacent sequences: A085730 A085731 A085732 * A085734 A085735 A085736 KEYWORD nonn,base,fini,full AUTHOR G. L. Honaker, Jr., Jul 20 2003 EXTENSIONS More terms from Reinhard Zumkeller, Jul 22 2003 More terms from Hugo Pfoertner, Jul 22 2003 STATUS approved

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Last modified July 23 16:47 EDT 2024. Contains 374552 sequences. (Running on oeis4.)