NB. p233

step2=: 3 : '({.y)+({:y)*i. >. (--/}:y)%{:y'

NB. f(N) with odd N
fo=: 3 : 0
a=: }. step2 (-:y),(y%%:2),1
b=: %: (-:*:y)-*:a
b=:(0=1|0.5+b)#b
4+8*#b
)

NB. f(N) with even N
fe=: 3 : 0
a=: step2 (>:-:y),(y%%:2),1
b=: %: (-:*:y)-*:a
b=:(0=1|b)#b
4+8*#b
)

f=: 3 : 0
if. 2 | y do.
	fo y
else.
	fe y
end.
)

f420=: 3 : 0
NB. <y> starting 1000 to look at
NB. N > 10,000,999
v=: y + i.1e3
+/420=(f"0) v
)