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A080696
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Piptorial numbers = product of first n pips or prime-indexed primes.
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7
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3, 15, 165, 2805, 86955, 3565155, 210344145, 14093057715, 1169723790345, 127499893147605, 16192486429745835, 2542220369470096095, 455057446135147201005, 86915972211813115391955, 18339270136692567347702505, 4419764102942908730796303705
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OFFSET
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1,1
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COMMENTS
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The numbers after the first always end in 5. This is obvious since all pips are odd and their product (excluding 5) = 2k+1 and 5*(2k+1) = 10k+5. Sum of reciprocals converges to 0.4064288978193657814428353009..
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LINKS
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FORMULA
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a(n) = Product_{k=1..n} prime(prime(k)). - Michel Marcus, Mar 15 2021
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EXAMPLE
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prime(prime(1)), prime(prime(1))*prime(prime(2)), ...
pip(1) = 3, pip(2) = 5, pip(3) = 11; piptorial(3) = 3*5*11 = 165.
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MATHEMATICA
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nn=50; FoldList[Times, 1, Transpose[Select[Thread[{Prime[Range[nn]], Range[nn]}], PrimeQ[ Last[#]]&]][[1]]] (* Harvey P. Dale, Jul 05 2011 *)
FoldList[Times, Table[Prime[Prime[n]], {n, 20}]] (* Harvey P. Dale, May 06 2018 *)
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PROG
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(PARI) piptorial(n) = {sr=0; pr=1; for(x=1, n, y=prime(prime(x)); pr*=y; print1(pr" "); sr+=1.0/pr; ); print(); print(sr) }
(PARI) a(n) = prod(k=1, n, prime(prime(k))); \\ Michel Marcus, Mar 15 2021
(Python)
from sympy import prime, nextprime
def aupton(terms):
prod, p, alst = 1, 2, []
while len(alst) < terms:
p, prod = nextprime(p), prod * prime(p)
alst.append(prod)
return alst
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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