Below are a few possibilities around Tachikawa station, 20-30 mins walk from the symposium venue.

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Some reference points:
★★★: most expensive
★★: relatively less expensive
★: least expensive

Palace Hotel Tachikawa★★★
http://www.palace-t.co.jp/english/index.html
http://www.palace-t.co.jp/index.html

Tachikawa Grand Hotel★★
http://tachikawa.khgrp.co.jp/

Tachikawa Washington Hotel★★
http://www.tachikawa-wh.com/

Tachikawa Regent Hotel★
http://www.tachikawa-regent-hotel.jp/

Super Hotel Tokyo-JR Tachikawa Kitaguchi★
http://www.superhotel.co.jp/s_hotels/tachikawakita/tachikawakita.html

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For more possibilities, the following search links would be useful if you're familiar with Japanese websites:
Jalan
Rakuten Travel
Ikyu.com