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 A332787 Negative-pan primes (see Comments). 3
 2, 3, 7, 11, 13, 17, 19, 23, 29, 37, 41, 43, 47, 53, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 227, 229, 233, 239, 241, 251, 257, 263, 277, 281, 283, 293, 307, 311, 317, 331, 337, 347, 349, 353, 359, 367 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Take a double-pan balance scale and name the pans "negative" and "positive". At each step, the question is: "Is there an unused prime that would balance the scale if added to the positive pan?" If the answer is yes, add that prime to the positive pan. Otherwise, add the smallest unused prime to the negative pan. The negative pan N can be fractalized, i.e., subdivided into NN and NP pans, where NN ={{2,3,7,11},{13,17,19,29,37,41,43},...} and NP = {{23},{199},...}. Can this fractalization be continued infinitely? LINKS Michael S. Branicky, Table of n, a(n) for n = 1..10000 EXAMPLE First division: 2 and 3 unbalance the scale (and go to the negative pan N), but 5 = 2 + 3 balances it (and goes to the positive pan P). Second division: 2,3,7 and 11 unbalance the N pan (and go to the NN subpan), but 23 balances it (and goes to NP subpan). MATHEMATICA a[1]=-2; a[n_]:=a[n]=Module[{tab=Table[a[i], {i, 1, n-1}], totalN=Abs[Total[Select[Table[a[i], {i, 1, n-1}], Negative]]], totalP=Total[Select[Table[a[i], {i, 1, n-1}], Positive]], l=NextPrime[Last[Select[Table[a[i], {i, 1, n-1}], Negative]], -1]}, If[ totalN==totalP, If[ PrimePi[tab[[-1]]]-PrimePi[Abs[tab[[-2]]]]==1, -NextPrime[tab[[-1]]], NextPrime[tab[[-2]], -1]], If[PrimeQ[totalN-totalP]&&FreeQ[Abs[tab], totalN-totalP], totalN-totalP, If[FreeQ[Abs[tab], Abs[l]], l, While[!FreeQ[Abs[tab], Abs[l]], l=NextPrime[l, -1]]; l] ]]]; Abs[Select[a/@Range[78], Negative]] PROG (Python) from itertools import islice from sympy import isprime, nextprime def agen(): # generator of terms used, d, nextp = set(), 0, 2 while True: if d > 0 and d not in used and isprime(d): used.add(d); d = 0 while nextp in used: nextp = nextprime(nextp) used.add(nextp); yield nextp; d += nextp print(list(islice(agen(), 65))) # Michael S. Branicky, May 12 2022 CROSSREFS Cf. A249031, A332341, A332788. Sequence in context: A173555 A086339 A333364 * A181173 A216277 A089174 Adjacent sequences: A332784 A332785 A332786 * A332788 A332789 A332790 KEYWORD easy,nonn AUTHOR Ivan N. Ianakiev, Feb 24 2020 STATUS approved

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Last modified October 3 16:48 EDT 2023. Contains 365868 sequences. (Running on oeis4.)