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 A289993 Primes p such that gpf(A288814(p)) < q, where q is greatest prime < p. 3
 211, 541, 631, 673, 1693, 1801, 2879, 3181, 3271, 3299, 3343, 3571, 3943, 4177, 4327, 4441, 4547, 4561, 4751, 4783, 4813, 4861, 5147, 5261, 5381, 5431, 5501, 5779, 6029, 6197, 6421, 6469, 6521, 6599, 6673, 6883, 6947, 7103, 7283, 7321, 7369, 7477, 7573, 7603, 7789, 7901, 7963, 7993, 8419, 8443 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For prime p in this sequence, b(p) = r*b(p-r) where b(m) = A288814(m), and r = gpf(b(p)) is some prime < q. We can say that prime p_n (n > 2) is of type k if gpf(b(p_n)) = p_(n-k). Prime gap p-q, and pattern of gaps p-r determines if p is in the sequence or not. Prime p is of type k > 2 only if p-q is one of the even indices of A056240 on which A292081 is defined (12,18,24,28,30,36,...), and if there is a prime r < q < p such that b(p-r) < b(p-q). LINKS David A. Corneth, Table of n, a(n) for n = 1..31763 (terms up to 5*10^6; first 544 terms from Robert Israel) David A. Corneth, PARI program. EXAMPLE p=211 is a candidate for inclusion because p-q = 211-199 = 12, and b(12)=35 is a term in A292081. Since r=197 is the next prime below q, p-r = 14 and b(14) = 33 < 35, 211 is in the sequence, of type 2. Conversely, p=809, which also has gap p-q = 12, is not in the sequence because the only number n > 12 for which b(n) < b(12)=35 is n=14, and p-14 = 795 is not prime. Therefore b(809) = 797*b(12) = 27895, and 809 is of type 1. MAPLE N:= 10^7: # to get terms before the first prime p>3 such that A288814(p) > N Res:= NULL: for x from 4 to N do if isprime(x) then next fi; F:= ifactors(x)[2]; p:= add(t[1]*t[2], t=F); if not isprime(p) then next fi; if not assigned(A288814[p]) then A288814[p]:= x; w:= max(seq(t[1], t=F)); if w < prevprime(p) then Res:= Res, p fi fi od: pmax:= Res[-1]: Primes:= select(isprime, [seq(i, i=5..pmax, 2)]): B:= remove(p -> assigned(A288814[p]), Primes): sort(select(`<`, [Res], min(B))); # Robert Israel, Oct 19 2017 PROG (PARI) see PARI link. - David A. Corneth, Mar 23 2018 CROSSREFS Cf. A000040, A056240, A288814, A288313, A292081, A001223. Sequence in context: A073102 A076167 A217620 * A300334 A032659 A098674 Adjacent sequences: A289990 A289991 A289992 * A289994 A289995 A289996 KEYWORD nonn,easy AUTHOR David James Sycamore, Sep 13 2017 EXTENSIONS a(30)-a(50) from Robert Israel, Oct 02 2017 Edited by Michel Marcus, Nov 15 2023 STATUS approved

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Last modified May 28 22:13 EDT 2024. Contains 372921 sequences. (Running on oeis4.)