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dik;saGkhye saJjJAyAm

दिक्संख्ये संज्ञायाम् ONPANINI 21050

Numerals, and nouns meaning quarters, compound with words in the same case when a tag.

And the resulting compound is a /tatpuruSa, but not a /dvigu. So we say —

saptarSayaH सप्तर्षयः "the Seven Sages" (the Big Dipper)
paJcAmrAH पञ्चाम्राः "Five Mangotrees" (a placename)
pUrveSukAmazamI पूर्वेषुकामशमी "Eastern iSukAmazamI", apareSukAmazamI अपरेषुकामशमी "Western iSukAmazamI" (names of villages)

Why do we say "in a tag"?

No compounding allowed when just any seven sages are meant —

RSayas sapta ऋषयः सप्त "seven sages"

But when the Big Dipper, or the famous Seven Sages of legend are meant, those are tags, so the compounding is allowed.

So who are those special seven sages?

vasiSThaH kazyapo 'thAtrir vizvAmitro 'tha gautamaH |
वसिष्ठः कश्यपो ऽथात्रिर्विश्वामित्रो ऽथ गौतमः ।
jamadagnir bharadvAjas te 'pi sapta-maharSayaH ||
जमदग्निर्भरद्वाजस्ते ऽपि सप्तॱमहर्षयः ॥
"vasiSTha वसिष्ठ kazyapa कश्यप and atri अत्रि
vizvAmitra विश्वामित्र then gautamaH गौतमः
jamadagni जमदग्नि bharadvAja भरद्वाज
they're the Seven Super Seers".

Why did you say "but not a /dvigu"?

Because saGkhyApUrvodv... >> affects only the next sUtra, not this sUtra. As Coulson says, otherwise this sUtra and the next would have been made into one. And then saptarSi would be singular by dvigurekav....