Yes, WHACK = hit. WACK = strange (as in wacky). Different words but WACK seems valid.
Same here TILES > TILLS > TELLS > SELLS > SEALS > SEATS > SLATS > SLATE
TILES > TIRES > TERES > TERTS > TEATS > SEATS > SLATS > SLATE
and all are in Collins
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Continue the following sequence → 2, 3, 5, 9, 17 …
Perhaps you could add just one or two numbers at a time.
I have a particular sequence in mind, and its fairly simple. However, i’m not entirely certain that it’s unique. So we might end up with more than one solution. I hope not, but who knows !!
1099511627777
17592186044417
Not next in the sequence, but they do appear …
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41st and 45th. But I might have mis-counted. 
Hopefully, someone (?) will post 6th and 7th
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They are … and at 50
562949953421313
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33 and 65…
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Well done Cat7
and WalnutTree
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Excel-lent !!!
Not quite …
Excellent - yes
Excel-lent - no
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It reminds me of the grains of sand on a chessboard puzzle. What would be the total after 64 squares using Don’s series?
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about 1.845x10¹⁹
That looks about right, using a rule of thumb.
Can anyone provide an exact answer?
As a teaser, on the 64 square there will be: 9223372036854775809
Corrected - pasting failed!
Is that the 64th term in Don’s sequence? Why is it not an odd number?
Paste failure! Now corrected. Will get teh final number later.
I agree your revised figure. I used no electronic calculating device, just pen and paper to check your figure.
The formula is resonably easy as it working it out on paper - I then tried checking it in a spread sheet knowing there is a 15 digit limit - which can be overcome.
My current aim is to do the sum using a spreadsheet, with the limitation, rather than calculate on paper. That is the “challenge”!
By the way, although I didn’t use an electronic calculator (or Excel etc), I just checked your figure against other figures already available on this thread. Cheating? Or just efficient maths?