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A109378 Semiprimes at Levenshtein distance n from previous value when considered as a decimal string. 0
4, 6, 10, 221, 1003, 22226, 100001, 2222245, 10000001, 222222223, 1000000006, 2222222227, 100000000013, 2222222222249, 10000000000015, 222222222222223, 10000000000000031, 22222222222222229, 100000000000000015 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

For positive n, the string length of a(n+1) is always the 1 + the string length of a(n). This sequence is infinite.

LINKS

Table of n, a(n) for n=0..18.

Michael Gilleland, Levenshtein Distance, in Three Flavors. [It has been suggested that this algorithm gives incorrect results sometimes. - N. J. A. Sloane]

V. I. Levenshtein, Efficient reconstruction of sequences from their subsequences or supersequences, J. Combin. Theory Ser. A 93 (2001), no. 2, 310-332.

EXAMPLE

a(0) = 4 = 2^2.

a(1) = 6 because we transform a(0) = 4 to 6 = 2 * 3 (a semiprime) with one substitution.

a(2) = 10 because we transform a(1) = 6 to 10 = 2 * 5 with one substitution and one insertion.

a(3) = 221 because we transform a(2) = 10 to the least semiprime 221 = 13 * 17 with 1 substitution plus two insertion.

a(4) = 1003 because we transform a(3) = 221 to the least semiprime 1003 = 17 * 59 with 3 substitutions plus one insertion and any smaller semiprime can be transformed from 221 in fewer than 4 steps.

a(20) = 10000000000000000001 = 11 * 909090909090909091, which is the least semiprime of Levenshtein distance 20 from a(19) = 2222222222222222222 from which decimal string we transform to a(20) with 19 substitutions and one insertion.

CROSSREFS

Cf. A001358, A081355, A081356, A081230, A109809, A109811.

Sequence in context: A136838 A234276 A274991 * A132149 A129235 A012903

Adjacent sequences:  A109375 A109376 A109377 * A109379 A109380 A109381

KEYWORD

easy,nonn,base

AUTHOR

Jonathan Vos Post, Aug 25 2005

STATUS

approved

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Last modified September 15 14:53 EDT 2019. Contains 327078 sequences. (Running on oeis4.)