help emacs-w3m key binding with F21 and F22

[Jue Kong <nwamtf@xxxxxxxxx>]

hello,

I want to bind the key F21 (Navigate backward) and F22 (Navigate
forward) to emacs-w3m with the code bellow:

(defun w3m-add-keys ()
  (define-key w3m-mode-map <f21> 'w3m-view-previous-page)
  (define-key w3m-mode-map <f22> 'w3m-view-next-page))
(add-hook 'w3m-mode-hook 'w3m-add-keys)

but generate error:

symbol's function definition is void: <f21>

can you give me some suggestion? thanks.

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