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 A251092 a(n) is the number of primes in the n-th group of consecutive primes among the odd numbers. 1
 3, 2, 2, 1, 2, 1, 2, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 2, 1, 1, 1, 2, 2, 1, 1, 1, 1, 2, 2, 2, 1, 1, 2, 1, 2, 1, 1, 1, 2, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 2, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Explanation: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25,... = Odd numbers    ^  ^  ^      ^   ^       ^   ^       ^         = Prime numbers   <---3--->    <--2-->     <--2-->    <-1->       = Primes beside other primes divided into groups of how many there are. Note that the first group (3, 5 and 7) is the only group of 3. Therefore the rest of the sequence only consists of 1's and 2's. Essentially the same as A175632. - Robert Israel, Mar 29 2015 LINKS MAPLE N:= 1000: # to use the first N+1 odd numbers L:= map(t -> isprime(2*t+1), [\$1..N]): Starts:= [1, op(select(i -> L[i] and not L[i-1], [\$2..N]))]: Ends:= select(i -> L[i] and not L[i+1], [\$1..N-1]): seq(Ends[i]-Starts[i]+1, i=1..nops(Ends)); # Robert Israel, Mar 27 2015 MATHEMATICA Length /@ Split[Select[2 Range@ 1200 - 1, PrimeQ], #2 - #1 == 2 &] (* Michael De Vlieger, Mar 20 2015 *) CROSSREFS Cf. A000040 (prime numbers), A005408 (odd numbers), A001097 (twin primes), A175632. Sequence in context: A152159 A341941 A090341 * A175632 A226481 A088435 Adjacent sequences:  A251089 A251090 A251091 * A251093 A251094 A251095 KEYWORD nonn AUTHOR Arthur Edward Chadwick, Mar 20 2015 STATUS approved

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Last modified May 10 20:33 EDT 2021. Contains 343780 sequences. (Running on oeis4.)