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 A097785 Counterexamples to the conjecture that an even, prime-indexed triangular plus 1 equals a prime or that an odd, prime-indexed triangular minus 2 equals a prime. 0
 497, 1771, 2279, 3161, 3487, 5149, 5357, 5993, 6439, 8129, 9451, 9731, 11477, 15049, 16469, 18337, 19901, 25879, 26333, 28681, 29159, 34717, 40187, 43069, 48517, 50401, 54947, 60379, 61073, 62479, 67529, 69749, 72011, 73537, 79001, 88829 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS In the entry for 496, Wells remarks that it is the smallest counterexample to the conjecture that an even, prime-indexed triangular plus 1 equals a prime, since 497 is not prime. REFERENCES David Wells, The Penguin Dictionary of Curious and Interesting Numbers. LINKS MATHEMATICA tri[n_] := n(n + 1)/2; tp = Table[ tri[ Prime[n]], {n, 2, 85}]; f[n_] := If[ OddQ[n], n - 2, n + 1]; Select[f /@ tp, Not[ PrimeQ[ # ]] &] CROSSREFS Cf. A096333. Sequence in context: A013685 A108157 A062676 * A145922 A145981 A210785 Adjacent sequences:  A097782 A097783 A097784 * A097786 A097787 A097788 KEYWORD nonn AUTHOR Alonso del Arte and Robert G. Wilson v, Aug 24 2004 STATUS approved

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Last modified January 24 01:05 EST 2020. Contains 331178 sequences. (Running on oeis4.)