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 A093515 Numbers k such that either k or k-1 is a prime. 14
 2, 3, 4, 5, 6, 7, 8, 11, 12, 13, 14, 17, 18, 19, 20, 23, 24, 29, 30, 31, 32, 37, 38, 41, 42, 43, 44, 47, 48, 53, 54, 59, 60, 61, 62, 67, 68, 71, 72, 73, 74, 79, 80, 83, 84, 89, 90, 97, 98, 101, 102, 103, 104, 107, 108, 109, 110, 113, 114, 127, 128, 131, 132, 137, 138, 139 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Original name: Transform of the prime sequence by the Rule 110 cellular automaton. As described in A051006, a monotonic sequence can be mapped into a fractional real. Then the binary digits of that real can be treated (transformed) by an elementary cellular automaton. Taking the resulting sequence of binary digits as a fractional real, it can be mapped back into a sequence, as in A092855. From M. F. Hasler, Mar 01 2008: (Start) The "Rule110" transform as used here involves a right-shift of the sequence before applying the transform as described on the MathWorld page. Robert G. Wilson v observed that this sequence contains exactly the indices for which A121561 equals 1. (End) From M. F. Hasler, Jan 07 2019: (Start) The correspondence of monotonic sequences with fractional reals mentioned in the first comment is not really relevant here: RuleX most naturally maps directly one characteristic sequence to another and thus one set (or increasing sequence) to another one. Interpreting the characteristic sequences as binary digits of a fractional real then yields a map from [0,1] into [0,1] rather as a consequence. Antti Karttunen observed that this seems to be the complement of A005381 (k and k-1 are composite). This is indeed the case: The characteristic sequence of primes has no three subsequent 1's. In all other cases of the 8 possible inputs for Rule110, the output is 0 if and only if the cell itself and its neighbor to the right are zero, which here means "k and k+1 are composite", and with the right shift mentioned above, the complement of A005381, i.e., numbers such that k or k-1 is prime (or: primes U primes + 1). We have actually proved the more general Theorem: Rule110 transforms any set S having no three consecutive integers into the set S' = {k | k or k-1 is in S} = S U (1 + S). (End) LINKS M. F. Hasler, Table of n, a(n) for n = 1..19998 (using prime(1..10^4)). Eric Weisstein's World of Mathematics, Elementary Cellular Automaton Eric Weisstein's World of Mathematics, Rule110 Elementary Cellular Automaton FORMULA {a(n)} = A000040 U (A000040 + 1), where A000040 are the primes. - M. F. Hasler, Jan 07 2019 MATHEMATICA Select[Range[2, 150], !(!PrimeQ[# - 1] && !PrimeQ[#]) &] (* Vincenzo Librandi, Jan 08 2019 *) PROG (PARI) {ca_tr(ca, v)= /* Calculates the Cellular Automaton transform of the vector v by the rule ca */ local(cav=vector(8), a, r=[], i, j, k, l, po, p=vector(3)); a=binary(min(255, ca)); k=matsize(a); forstep(i=k, 1, - 1, cav[k-i+1]=a[i]); j=0; l=matsize(v); k=v[l]; po=1; for(i=1, k+2, j*=2; po=isin(i, v, l, po); j=(j+max(0, sign(po)))% 8; if(cav[j+1], r=concat(r, i))); return(r) /* See the function "isin" at A092875 */} (PARI) /* transform a sequence v by the rule r - Note: v could be replaced by a function, e.g. v[c] => prime(c) here */ seqruletrans(v, r)={my(c=1, L=List(), t=0); r=Vecrev(binary(r), 8); for(i=1, v[#v], v[c]

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Last modified May 29 05:47 EDT 2020. Contains 334697 sequences. (Running on oeis4.)